Altruism is the first and greatest meme
The fact that groups celebrate selflessness is practically tautological.
People are altruistic because they know that others will emulate them, essentially returning the favor
Property and Selfishness
There are two sides in the debate on the origins of altruism: one that insists we're selfless, the other that we're selfish. However, the distinction between selfishness and non-selfishness is biased by our socially-acceptable notion of human goodness. We think we're selfish, but that's just because we have norms where we don't ask for people's charity. If we were selfless, all of our friends and family would have the same amount of money, or at least everybody would often rebalance so that everybody was roughly free from need in the same amount.
When children play, we often police them to make sure they share their toys. And yet, as adults, we would never share our property with the same frequency. It's just that adults know not to bug each other for their stuff. As adults, we are as selfish as we were in our natural state as children.