"It's mysterious that human beings among all mammals are so hyper-social that our brains are wired to help other people, even strangers." said Paul Zak of the Center for Neuroeconomics Studies at Claremont Graduate University. "Economists have always been shocked by unselfish altruism, and now we have a reason for it: It feels good to do this."
This study supports the idea of "pure altruism" -- that people take action even if the behavior is not explicitly in their own interest.
As for why humans would develop a desire to help others, the researchers can only speculate. One basic is that early humans lived in small groups, where survival of the group helped your own cause.