Friendships form via shared context, not shared activities
I think this phenomenon is part of the reason it's difficult to form friendships as adults. The shared context that the article mentions requires a lot of time around your circle of people, just "hanging out". It's tough to make that time as an adult with a career, family, etc.This may be why college friendships can be so strong – often a lot of time is spent living and working around friends by default, without needing to schedule it.