One thing I’ll add is that altruism toward random strangers does happen, with lower frequency, but it’s a very different beast than doing nice things for your friends. It’s kind of unnatural.

In my experience, “practicing random kindness and senseless acts of beauty” has its own weird emotional charge that’s hard to explain. I think part of it is that people react very strongly to unexpected/undeserved kindness (and I’m talking things above and beyond normal politeness), so the empathic hit of vicarious happiness is pretty good. And I think maybe part of it is that it gives you this really interesting sense of power over the universe and over your own human nature to do something nice with no expectation of reciprocity at all, just being nice out of spite (well, the opposite), as it were.