It can be summed up best by looking at these two quotes together.

From The God Delusion:

"Do you really mean to tell me the only reason you try to be good is to gain God’s approval and reward, or to avoid his disapproval and punishment? That’s not morality, that’s just sucking up, apple-polishing, looking over your shoulder at the great surveillance camera in the sky, or the still small wiretap inside your head, monitoring your every move, even your every base though."

From The Selfish Gene:

"Any altruistic system is inherently unstable, because it is open to abuse by selfish individuals, ready to exploit it."