Just to present an opposing view:
People have presented plenty of arguments against utilitarianism (here and elsewhere), but:
- There are responses to those arguments (but there may also be responses to those responses, with subsequent responses)
- Those arguments often involve examples that are very extreme and completely detached from reality, which doesn't mean it's a bad principle for non-extremes.
- People like to consider utilitarianism in isolation, but we should ask how it compares to other moral philosophies.
- If pressed, one could combined utilitarianism with e.g. some rights-based morality. You don't necessarily need to treat maximising well-being as the ultimate moral authority to appreciate that maximising well-being is generally a good thing to strive towards.
There is no perfect moral principle because reality is messy, but utilitarianism is one of the best, or the single best, principle we've been able to come up with.
At least as far as I've seen, every prominent alternative to utilitarianism seems to invoke ideas that seem unjustified, poorly defined or arbitrary, they can (and do) fairly-trivially justify utterly disgusting and atrocious acts, they lead to drawing arbitrary lines, or they in some form appeal to increasing happiness or avoiding suffering (utilitarian ideals).
To consider some arguments and responses:
"Utilitarianism may lead to killing a few to increase the happiness of the many"
Response: for this, the downside to the oppressed is typically far more severe than the upside to the advantaged, the upside for the advantaged may not be that advantageous (many people are unhappy living in an unequal society, even if it's in their favour), the tables could turn (due to e.g. revolution, which is less likely under equality).
One may judge suffering to be exponentially worse than happiness is good, and consider happiness to have diminishing returns, so it would be implausible for a scenario like the hypothetical to actually happen.
"There is no objective way to compare utility/happiness across different people"
Response: if you're going to charitably give money to a stranger, would that be to a billionaire, or to the homeless person who can't afford food? One doesn't even need to make a case there, because practically every person is already able to compare utility/happiness. Objective and quantitatively don't apply to emotion in general. Evaluating someone's emotional state is inherently subjective and qualitative. But that doesn't mean you can't strive towards consistency.
If you want some quantifiable measure (which is still subjective), consider how you'd feel about living in one person's life, compared to another. Let's say you'd live as a homeless person for 5 years, and then you'd live as a billionaire for 5 years, after which you'd go back to your normal life. If you can have $10 at some random point during that period, would you rather have it while homeless or while a billionaire? A slight variant on that: how much money would you choose to transfer from the billionaire to the homeless person? How much longer would you be willing to be homeless to spend more time as a billionaire?
This may not be a perfect comparison, because you may not know how it would feel to live as someone else, especially in the long term, and especially given that life experiences, physiological variation and mental conditions significantly affect one's experience. But it could provide a lot of useful insights into how well-being compares in different circumstances.
"Utilitarianism would demand that you donate all your possessions because consideration for yourself and those around you is equal to consideration for those living in extreme poverty, and your money would best serve them"
Response: one may say that not every action you perform should serve the greatest good. We are emotional creatures that are strongly driven by our own interests, and this can't really be ignored.
If you're really inclined to argue this from utilitarianism, one might say that a moral philosophy which does not give some preference to oneself and those around you cannot generally be adopted or maintained, for reasons given above. Thus what would provide the greatest increase in well-being is if utilitarianism gives some preference to one's own interests. But maybe this is a bit contrived.
I've also addressed the idea of selling all your possessions in an earlier answer, although that doesn't specifically mention utilitarianism.