In all seriousness, people probably do more favors for attractive strangers. And HN is probably populated with average-looking people, at best. In some ways, this means that the stories posted here will be of true altruism.

On the other hand, if you asked a bunch of great-looking people what strangers had done for them, you'd get a bunch of stories about people who were very kind (but perhaps not entirely altruistic in their intent).