"Are the poet’s desires, Are THINE OWN desires ‘neath a thousand guises?
A "guise" is a "disguise," or a "mask."
People often represent their personal desires as something other than a personal desires.
Consider the following two sentences:
- I, personally, would prefer that you not rape me.
- Rape is wrong. It would be
unethical if you raped me. Additionally, you would burn in hell for all eternity if you did it.
Note that statement 2 is often more persuasive than statement 1. That rape is "
wrong" is not true according to Nietzsche, but is usually more persuasive, than just "please don't rape me, I would rather not have sex with you."
Nietzsche's view of the universe is as if the universe is made of rocks and some laws of physics governing the rocks. There is no god. The rocks do not care if you rape, murder, steal, etc... The laws of physics do not care if use an axe to lop off someone's head. There is no morality. There are no ethics.
As such, all statements about ethics and morality are really just statements of personal preference in disguise.
- It is unethical for a person to rape another person.
- It is immoral to rape
- It is ethically unconscionable to rape someone.
- It is an evil act to rape someone.
- Rape is bad.
- It is wrong to rape.
- People should not rape.
- I would prefer that people not rape
- I want everyone to not rape anyone else.
In Nietzsche's view of the world, people hide their personal desires using many different masks.
All claims about morally are personal preferences painted to look like something else. When it comes to ethics, people portray their own personal preferences as if they were laws of physics written into the very fabric of the universe. "I would prefer to not be physically forced to have sex," becomes things like, "The very fabric of the universe will dislike you extremely if you rape a another person."
You asked, "what is wrong with self-hiding and putting a desire in a disguise?"
The answer is that Nietzsche is passionately in love with the pursuit of truth. Nietzsche wanted to try to be as truthful as possible, and wanted other people to try to be as truthful as possible. Confusing your personal desires with the laws of physics governing the universe is something that disgusted Nietzsche. He was personally disgusted by deceptions and obfuscations of truth. If you, also, are a lover of truth (most philosopher's are), then you might be able to see why Nietzsche abhorred such things.