I'm copying my own answer to remember it, even if I'm not thrilled with it:
Socrates said that the question of why one should be good was as silly as the question of why one should be healthy.
Goodness is part of wisdom: understanding the world outside one's own mind, understanding consequences, and understanding that you giving a damn can make others' lives better even after your life inevitably decays away. Your morality is part of society's health.
In line with what Case said above, a lot of "right" and "wrong" is pragmatically socially constructed. What we call right and wrong is partly what society has come up with that works. It's pro-social, and not entirely arbitrary. (There is more to it than that. There is morality beyond social convention, and persons can disagree on morality without being amoral.)
Now, someone can be a free rider, so long as they get away with it.
This entry was originally posted at http://philippos42.dreamwidth.org/14626