Human's struggle against a society.
Societies were constructed to corral man's tendency toward self-preservation as well as self-gratification. Religion does it one better by introducing a fear factor: if you're not good you're not going to get a treat when you die. And you better not be having any fun while you're being good.
Don't believe me about the struggle? How many people you know that drive the speed limit, stop for yellow lights, return the grocery cart, switch lines in the bank or grocery store, file taxes with complete honesty?
It's not that we're all evil, although some certainly are. It's that we're impatient, or we have a feeling that we're not being treated fairly. Or we're upset that someone is getting something they don't deserve.
We bemoan welfare mothers but cheer on billionaires who receive more tax breaks than we'll ever pay out in welfare.
I'm afraid the percentage of truly altruistic people is quite low.