Does Altruism Matter?

The other day on the subway, I gave up my seat for an older woman.  Afterwards, I thought about why I had done it.  Some was out of what you might call altruism, an actual concern that her feet probably hurt more than mine did.  Some was to maintain my community as one I want to live in.  When I'm pregnant or old, I want people to give me seats, so I'd better do the same for such people now.  And some was because I want people to see me as a good person, the kind of person who gives seats to old ladies.

Some people like to analyze whether altruism is real.  "Aha," they say, "there's no such thing as pure altruism!  You're really doing it out of selfishness!"
...so?  The woman who got my seat on the subway doesn't care.  

You can help other people because you think it's morally right, or because it feels good, or for the tax break, or to impress your friends.  The result is the same.  Do it for whatever reason works for you.