Because we want to

Theists wonder why we atheists don't run around killing everyone in sight, and atheist moral objectivists seem to wonder the same thing about subjectivists such as myself. The idea, I suppose, is that without the idea of divine punishment, or the illusion of an objective ethics, there's no "reason" to be good.

It's as if our modern notions of "good" require so much self-sacrifice, so much self-denial, that it seems incomprehensible that one could be good unless one is forced to be good; it seems inconceivable that one could want to be good.

I give half the blame for this prejudice to Christianity, the other half to Kant. (Kant at least for writing so incomprehensibly that his readers consistently misinterpret him.) It's not surprising that Christians would feel this way: The morality one can extract literally from the Bible is so thoroughly self-denying and self-sacrificing that no rational atheist would ever follow it. Happily, no one actually expects literal Biblical ethics any more. Kant deserves some blame for defining "morality" as an act taken without any self interest. Now I'm not a Kant scholar, and it's entirely possible that Kant (a very smart guy) was attempting a proof by contradiction or some such, but millions of people have read him as establishing that self-interest cannot by definition be the foundation of morality.

Humanism takes a fundamentally different tack: Humanism is about what humans feel is good. And the crux of the biscuit is that human beings are naturally empathic: We do in fact feel bad when our fellow human beings suffer, and we do in fact feel good when our fellow human beings are happy. We didn't reason this out, this is just a fact about how we feel. In much the same way, you could absolutely convince me that eating a pound of Brussels sprouts every day would make me immortal and give me super-powers, you could convince me to eat a pound a day, but no amount of logical argumentation can ever make them taste good. Feelings are not the outcome of conscious deliberation, they are simply facts about our minds.

If people did not usually feel empathy, theists and moral objectivists would simply not argue that empathy "ought" to be the cornerstone of our morality. The idea that empathy is "objectively" true is a conclusion based on the evidence that most people actually feel empathy. I've written elsewhere about why the conclusion of objective truth is unjustified; here I'll just say that if the evidence of common feeling were sufficient to establish objective truth, then this selfsame evidence can also directly justify our social and legal rules, without the mediation of objective truth.

All atheists—objectivists and subjectivists alike—believe there is no supernatural enforcement of ethics. All atheists—objectivists and subjectivists alike—behave as they do because they freely choose (at least free of supernatural coercion) to do so. Even the staunchest atheist ethical objectivist must freely choose to behave according to her beliefs about objective ethics. And even a theist must freely choose to read more-or-less Humanistic ethics back into his Torah, Bible or Koran.

There's a simple answer to the theists who wonder why atheists behave in a good manner: We behave in a good manner for precisely the same reason that theists allow Humanist ethics to perform such textual and metaphorical violence to the primitive, savage and brutal ethics literally described in the Bible: Because those ethics represent how we actually feel about what is good. That we feel something is good is a necessary (but not always sufficient) reason to do it. And that's true whether we just choose to do what we want, or choose to construct an "objective" morality that sanctions what we want, or reinterpret scripture to sanction what we want.