[dvergeylen]

>The common denominator is to have shared spaces where it's expected to be among strangers' presence, and for those strangers to eventually become repeat guests in a person's life. That's the maximally comfortable scenario for inducing social behavior and it's responsible for eons of human social history.

This is spot on. It's why you meet so much people during your high school / college years. You're among strangers' presence, while attending to class, which makes a natural topic of interest between the people involved.