I agree with ruveyn (STOP THE PRESSES!)...
...to an extent. (Never Mind!)
We are all, inherently, selfish creatures. Our bodies present particular needs and we spend our lives attending to these needs. That selfishness, can, of course, be channeled constructively, particularly where we develop the cause-->effect-->benefit reasoning that allows us to contemplate a larger, longer-term benefit that arises from an activity that does not present a short term gain. Reduced to purely financial terms: I could spend this $100 on a fancy dinner; or I could use this $100 to buy groceries and eat for a few weeks.
In the more complex world of social relationships, our participation can still be viewed in economic terms. I "invest" time in my friends: pursuing mutual interests, offering my expertise to help them when needed, doing small acts that cause them to feel gratitude. In return, then are willing to spend time with me, and help me when I need it.
Even altruism, a intent to do "good" with no expectation of return, may well be fuelled by what Aristotle referred to as, "the greed for virtue." (Though, in truth, Aristotles conception had more to do with "eudaimon" than with virture as we conceive of it in the concept of altruism.)
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--James