Committing crime can result in punishment by the judiciary.
Assuming extreme skepticism and that there is no flawless proof of an absolute goodness, are there any reasons that why one should not do criminal activities, if doing so can lead to a prosperous, luxurious or (biologically) happier life?
In the same spirit, should I not discourage crimes as there is no such absolute concept (not discouraging does not necessarily mean encouraging)?
Why or why not?
You could start by discouraging other people to do crimes if only because you don't want to be the victim of a criminal act. People around you will probably do the same, and this will establish a balance of common interest for a peaceful society.
See Kant's categorical imperative for a justification of why you can't expect to have rights others don't have, or Rousseau's concept of social contract for how the respective and mutual interest of each citizen for a peaceful society results in your own interest to abide the law.
Revenge!
This can be mathematically modelled. What is the optimum strategy for a society with complete libertarian freedom, i.e. no central authority? (Actually, it was single-celled organisms, but the same maths).
Tit-for-tat is the obvious strategy. You hurt me, I hurt you back.
It turns out that forgiving tit-for-tat works better in a situation where errors may be made (in other words, accidentally harming another, because of bad communications or circumstance). You forgive the first transgression. You retalliate (harder?) if it happens again.
Multiple forgiving works better still, but is unstable in the presence of evolution. It makes it possible for criminals to prosper when they discover how much they can get away with. With cells, one might call this cancer.
FWLIW, forgiving tit-for-tat is a strategy used by a lot of business people. You don't really know who you are dealing with when contracts are simply honoured as expected. (On the internet, nobody knows you're a dog!) You find out, only when things go wrong. The first time around, you accept what you are being told (if it's credible), and form a view as to how hard the other party is working to carry out the contract or repair the damage. If you suspect they aren't trying, or if the problem recurs, you stop doing business with the other party.
Employing lawyers usually isn't worth it.
This is related to the more general concept of social contracts, states, power hierarchies, and such. There have been several philosophers who have thought a lot about this. A good starting point would be to check out Hobbes' Leviathan, but there are also the contemporary Two Treatises of John Locke which are somewhat of a contrast to that. Anther philosopher would be famously Kant with his Imperative.
From a 10.000 km perspective, the gist is this: without any kind of law or power structure at all there is no crime, since crime is defined to be something which breaks law. But in that state, humans fall back to a base behaviour which - depending on whom you believe - is less than savoury. If you believe Hobbes, this would mean that the "law" of strength would rule; stronger people would roam the lands and just take whatever they want; there would be no law in our modern sense, and hence no crime, but many people would be very very unhappy (or dead).
The next step would be for the strongest humans to become chiefs or kings - they would gather lesser people around them and rule them by force. They themselves would be outside of law; they would be the law-giver. In return, they would protect the weaker ones from other tribes. This can work, and has for many thousands of years, but is obviously a little fickle and depends a lot on luck, especially regarding the character of the chief.
More modern and arguably refined methods then include a proper, written law, with no person above it, and some kind of structure (e.g. separation of powers) to protect it from individuals.
Thus, all we have today in the civilized world, including law, power structures and so on, follows from the need to get away from a barbaric state of affairs.
To answer your question: no, directly speaking, except for the punishment attached to the laws, there is no particularly reason to avoid crimes if you subscribe to the particular world view that the stronger should be allowed to take whatever they want and that (at least the existing) states, law, etc. are superfluous or illegitimate. Crimes only exist in the presence of law; if you deny law, then the concept of a crime simply does not apply to you.
On the other extreme, some people enjoy a less stressful existence very much and prefer to never take anything by force. For them, following the law (i.e., not doing crimes) is straightforward for the simple reason that if everyone does it, everyone gets to live a stable life.
The real world falls somewhere in the middle - the majority of people seems to be fine with following the law, but some prefer not to; many laws are very accepted and considered "good", some not so much; plenty of people decide that in some cases they can readily break the law (think of speeding), assuming that they can live with the consequences, and so on and forth. Some countries tend to the one side, some to the other. Life is messy.
Crimes are not arbitrary. While there are exceptions (so called "victimless crimes"), the general reason why an action is deemed to be a crime is because it harms (or has a likelihood of harming) other people or society. Thus, the reason why you should not commit these crimes is because it's wrong to harm others.
Crimes are generally considered to be codification of ethical principles held by the society as a whole. E.g. killing is considered very bad, so we declare murder to be a crime with severe punishment (which we hope will serve as a deterrent, but also satisfies a psychological need for retribution).
Even victimless crimes can be judged by these ethical principles. For example, suicide violates the principle that life is valuable and no one has the right to terminate it, even the person themselves.
Furthermore, in our modern, highly-connected society, many actions that may initially seem to be private have network effects. For example, drug use may seem like it only harms the user. But if they overdose, emergency medical help will likely be invoked, which costs the community. If medical attention is needed and they can't afford it, this cost will also be passed on. They also cause grief to family and friends. As the saying goes, "no man is an island" -- what you do almost always affects others. If these are negative impacts, it's not ethical to cause them intentionally when they can be avoided. If these are more than mere inconveniences, they may be made illegal.
Have you heard of the Prisoner's Dilemma? In some sense, society can be thought of as a large-scale version of such- maybe you would be a little better off if you committed crimes towards your neighbor, but probably both of you would be pretty bad off if you both committed crimes towards each other. There's I think some pretty good reasons which apply that say one shouldn't defect in the Prisoner's Dilemma, even if one is truly selfish. I think (though take this with a grain of salt, I'm no expert) that this is the basis for Kant's categorical imperative.
What's funny is that if you're in a society of people that do defect, then it seems that everyone does better if the payoffs for defectors go way down (because then everyone cooperates easily, and does better than if everyone defected). This is what the law essentially does, makes payoffs go way down for defectors (criminals), and is a good reason why you might like having laws even if you're perfectly selfish.
Utilitarianist answer:
Because, nicely enough, being cooperating is the best way for everyone to gain in the long term. Harming other people may put you in an advantegous position in the long term, but in the long term, the total productivity would be less.
In essence, trying to becoming in conflict with other people, simply for conflicts sake would in turn lead to a less productive society.
There was a nice free browser game modelling this. It's called The evolution of Trust.
Sure, there's a reason: long-term self-interest. Crimes are like taking the short-cut to everything in life. This would be fine -- if you were perfect, but you're not.
Taking the "long way" is how positive growth happens. Consider a vine's strategy to life vs. a tree's. Both compete for the same sunlight, but one is investing for the long-term. The tree offers secondary benefits for its efforts (shade for animals, greater oxygenation, sanctuary for birds). These secondary benefits which help others ultimately come back to help the tree.
Same with helping an orderly society.
It is a tenet in psychology that people behave the way they do because it was the option that made them feel best at the time, overall.
That is easy to see as the reason why people choose short-term pleasure even though it hurts them long-term, or why they seek an advantage that is unfair even though they generally would not think of themselves as bad persons.
It is harder to see why people sometimes choose to behave in a way that diminishes short-term or even overall pleasure. The answer is that this is still the behavior that makes them feel best in that moment. Donating to a charity, going out of one's way to help somebody, forfeiting the dessert still makes them feel better than the alternative (and sarcasts would add: especially if they can talk about it a lot afterwards). Freud would have said that they have a strong super ego which acts as an agent controlling immediate desires. Ignoring this agent's objections causes a discomfort stronger than any pleasure the individual would experience. The net balance is towards "being good".
The answer to your question is simply: People will always do what makes them feel best. (This tenet is actually true even if you believe in God or an absolute morality of any other sort.) We humans are social beings; we live and thrive in communities. This social life involves a lot of explicit and implicit promises, liabilities, obligations etc. We are free to ignore them and spoil the relationship to other people. If we can live with that for the sake of the benefits we have from our selfishness, we will. If the other people are more important to us than our own advantage, then we won't. It's that simple.
The Golden rule means it is unethical to commit crimes if we would not want to be the victim of such a crime. This is not quite the same as wanting to protect yourself from crime, just a logical consequence of assuming that you are not in an ethically privileged position. Neither is it about "goodness", just logical consistency.
Wanting to be rational and non-hypocritical is a reason not to commit crimes (although you could view it as your self-image being the punishment). I would argue that rational consistency and avoiding hypocrisy leads to a happier life, but that rather depends on your values.
There are plenty of negative consequences for crime that do not stem from the state.
Your premises beg the question a little. You're trying to say "what if we disregard state punishment", but how can you disregard it? There is always a risk that sooner or later the crime will be discovered. People have been punished decades after when victims or witnesses suddenly came forward or new forensics techniques revealed what was thought unknowable.
You also say "if doing so will lead to a prosperous life", but no one has a crystal ball. How can you know what will lead to a prosperous life? You can only reason about probable short term gain, which is not guaranteed for any crime.
Finally, even if you have no reason not to commit crime, you still have reasons to discourage crime:
Besides state punishment, are there any other reasons why one should not do crimes?
Your question is non-sensical because no one ever commits a crime or crimes, whereby whether or not a crime has occurred is based on the assertion of a crime having occurred rather than the assertion being known as a matter of fact; there is never anything inherit in anyone's commitments that qualify one or more acts by the said anyone as criminal; nature does not have anything in it that inherently right or wrong.
With that being said, one should do his or her best from a claim of having committed a crime from being asserted against himself or herself.
In relation to that paradigm of thought, the answer to your question would then be the following: No, relative to rational egoism, because it is a biological imperative for an individual to enhance one's socioeconomic status relative to the monopoly on violence as much as possible, as often as possible in order for the said individual to reduce its pain and suffering as much as possible, as often as possible relative to the pleasure principle.
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