Friendships form via shared context, not shared activities | Bill Mei

I think this is spot on.

That’s why friendships formed via shared interests often feel shallow. It’s because a real friend is someone who has things that can’t be listed on a résumé: trust, loyalty, love, belonging, safety, and a sense that they’ll back you up when the times get tough.

Real friendships don’t form via shared interests. They form via shared context.

This reminded me of Al Franker's comment

My Dad is fond of saying that kids don’t need quality time with their parents — they need quantity time.

Shared interests can be the beginning hook, but your long-term friends usually have shared a lot of experiences with you. This is probably one of the reasons it is so common to establish friendships with those you work with. You spend a lot of time with them!