Mean people

I'm beginning to think that there are people who go through their lives being mean, nasty, and generally unpleasant as a rule, not as the exception. I'm bumping into these types a bit more often than I would like to, certainly, but what I really don't understand is why anyone would deliberately choose to be this nasty?

I saw a bumper sticker the other day that sums it up nicely:
"Life is short. Don't be a dick."
Suppose they sell them by the caseload?