Why do people cooperate? To feel good? To avoid feelings of guilt?

Theory 1 is that most people cooperate because it feels good to do it. And there is some MRI brain imaging data that shows activity in reward-related regions of the brain when people are cooperating.

Theory 2 is about that people try to do good because they don't want to feel bad. That is the idea behind guilt aversion

http://uanews.org/node/39831