[pfdietz]

Altruism isn't common because we're created by evolution, and nothing is created by evolution without a selfish (for the genes) payoff. Supposed altruistic behavior in evolved creatures always has a selfish alterior motive (group selection, tit-for-tat payoff, for example).

[lazyasciiart]

So the answer to your question "why do you think altruism is common" is still what are you talking about?