Could be a lot of reasons. But most of all, these-
They are real. Not fake (beyond the minimal amount needed to survive, I mean).
They can either talk or listen. Both are desirable traits, at different times, with the same people sometimes.
They want to keep in touch, and do.
They are not shy about sharing good or bad news-related to the first quality.
They have extra-curricular interests of some kind.
They are opinionated, in a good way. I am not really into politics or celebrities, but anything else interests me, listening to what people may have to say.. about education, health, poverty, learning techniques or experiments, technology in language I can understand, and entrepreneurial ventures that they have tried.
A world-view of some kind, about the world we live in, is always welcome.
