We honor such behavior even when we cannot explain it, even when we are unable to emulate it ourselves. The man who jumps off a brdge to rescue a woman struggling in the freezing water below is a hero, not a fool, and the extreme risk to his own life, incurred for a stranger, is completely admirable. We mutely sense that if we could all live like this, the world would be better. No theory of optics, of seeing, of the brain can explain the concept of beauty. There is nothing empirical about beauty, yet we have wide ranging agreement that certain things are beautiful.
Altruism is beautiful.
It is a beautiful idea, that inspires us when other people act on it and makes us feel very good about ourselves when we act upon it. That's all we need to know.