Because we are capable of love
...or why economists will always be wrong.
This ought, of course, to be a full-blown essay, but it'll be a long time before I get around to that so think of this as the skeleton of an essay and you can fill in the blanks yourself.
I read way too often the idea that it's puzzling and irrational for people to cooperate, but it isn't. Cooperation and altruism only look irrational if you assume that emotion holds no value. Read 'Don't Shoot the Dog' by Karen Pryor and learn why 'might' only sometimes kind-of rules. Every example that appears to be such a mystery to economists and researchers can be explained very simply--because we are capable of love.
Cooperation and trust are (for the logical crowd) the way we get repeat business and one of the best ways to get things done.
But cooperation and trust also happen because:
--We want to think well of ourselves
--We care about people in general
--We want others to think well of us (for reasons other than making money off them)
--We want to be worth loving
--We want others to be worthy of our love
But because we won't recognize these things or include them in 'rational' analyses, we are vulnerable to free riders, those who take more than their 'fair share,' (well, we rationalize, they're just being rational. How can we fault them for that?').