To paraphrase Scott Adam's newest book, capitalism gives us incentives to serve others and help them to help ourselves.
And selfishness is the default of human nature - see a baby screaming when hungry or cold, toddler tantruming when they don't get what they want.
When we have all of our needs met, we can become more altruistic - hence all the wealthy who turn to philanthropy. All the other common cases are either social obligation (helping someone who helped you so they'll help you in the future) or familial altruism which is programmed in (helping children, siblings, parents). That's why many charities say help your neighbors (social obligation) or your religious siblings (familial).