Utilitarianism seems self-evidently true. You can disagree with the particulars, but the essence is correct—happiness and pleasure are good, while suffering and pain are bad. The utilitarianism of Mill is essentially identical to the hedonism of Epicurus. Hedonism, like utilitarianism, got a bad rap because people didn't understand it. Epicurean hedonism resembles Buddhism in many ways and contrasts starkly with the debauchery and immorality usually associated with the term “hedonism.” Both utilitarianism and hedonism, properly understood, are basically identical to the scientific ethical realism of Sam Harris, which he presents in The Moral Landscape. In my opinion, such approaches to ethics are fundamentally the same, regardless of the distinctions that their proponents tend to make between them. I refer to these tendencies broadly as “utilitarianism.”
Happiness and pleasure are good. Pain and suffering are bad. That's self-evident. It's wrong to punch someone in the face for no reason because it causes pain. When we say such-and-such hurts or causes pain or suffering, we just mean that we don't like it. When we say that such-and-such feels good and causes pleasure or happiness, we mean that we like it. The concept of pleasure/happiness is entangled with the concept of goodness. That's the essence of preference utilitarianism: the distinction between good and bad is a preference statement, indicating that one pleases us and the other displeases us—one causes pleasure and the other causes displeasure, whether that pleasure is psychological, emotional, physical, or otherwise. The concept of pain/suffering is entangled with the concept of badness. The word good has no meaning apart from happiness and the word bad has no meaning apart from suffering. Any theory of ethics that doesn't put this at the forefront is bogus. I believe this is the core claim of utilitarianism.
It is especially bewildering that the left is now so hostile to utilitarianism. Utilitarian ethics, when properly applied, tends to lead in the direction of libertarian socialism and social democracy. Democracy, liberty, and equality tend to emerge as the goals to be achieved by the polity. Traditional conservatives adamantly rejected utilitarianism precisely because it had such egalitarian, progressive, and libertarian implications.
I am a utilitarian but not to the exclusion of all alternative theories of ethics. I don't think that all the various theories of ethics are mutually exclusive. There are basically three approaches to ethics. Consequentialism emphasizes the importance of the consequences of actions, deontology emphasizes the importance of rules and obligations, and virtue ethics emphasizes the importance of cultivating virtue of mind and character. Any coherent and rational theory of ethics must simultaneously be all of the above.
If one adopts a synthesis of Larry Arnhart’s Darwinian natural right (a variation of Aristotelian ethical naturalism) and utilitarianism, then one can easily reconcile the three approaches. As a species, humans share human nature in common—have a common or shared set of values which we have inherited from our ancestors through evolutionary processes. Certain axiomatic values are ingrained in us by nature because of their evolutionary function. The most basic axiomatic value is our preference for pleasure over pain, our desire to seek happiness and avoid suffering. We have this value ingrained in us by nature because it was necessary for human survival. The process of natural selection weeded out individuals who didn’t try to avoid suffering because they simply weren’t suited for survival. A common human nature emerged from evolutionary processes. Natural selection is the blind god or blind watchmaker that chiseled away at our ancestors until it produced modern man. There is a universal basis for human ethics found in our shared human nature and a characteristic of that nature is to identify happiness and pleasure as good and suffering and pain as bad—humans naturally have a preference for good and a dislike for bad, but those designations really indicate nothing more than the preference itself. What emerges here is a preference utilitarian natural law theory.
We may then incorporate rule utilitarianism into our scheme and say that the right course of action is that which tends to maximize happiness and minimize suffering. This is still consequentialism, but now it is also deontology. Rule utilitarianism does not contradict act utilitarianism, but is merely a variation of it. Why is the rule good? Because the act of following the rule tends to maximize the probability of achieving our preference, which results in happiness or reduced suffering. Furthermore, we may say that the most appropriate thing to do is to develop a habit of conforming to such rules as have a tendency to maximize happiness and minimize suffering—we ought to cultivate virtue in order to maximize the probability of achieving the greatest amount of happiness possible. This theory of ethics reconciles act utilitarianism, rule utilitarianism, and preference utilitarianism to one another and reconciles such consequentialist approaches to deontology and virtue ethics. We are approaching a comprehensive and coherent theory of ethics.
If you want a more comprehensive overview of my theory of ethics, check out my 3-part series on Ethics and my essay Against Anti-Abortionism.