http://greatergood.berkeley.edu/topic/altruism/definition

Evolutionary scientists speculate that altruism has such deep roots in human nature because helping and cooperation promote the survival of our species. Indeed, Darwin himself argued that altruism, which he called “sympathy” or “benevolence,” is “an essential part of the social instincts.” Darwin’s claim is supported by recent neuroscience studies, which have shown that when people behave altruistically, their brains activate in regions that signal pleasure and reward, similar to when they eat chocolate (or have sex).

This does not mean that humans are more altruistic than selfish; instead, evidence suggests we have deeply ingrained tendencies to act in either direction. Our challenge lies in finding ways to evoke the better angels of our nature.