Shared stressors are how people form deep and lasting bonds. You have to endure an ordeal with someone. The war journalist Sebastien Junger wrote a whole book

Shared stressors are how people form deep and lasting bonds. You have to endure an ordeal with someone. The war journalist Sebastien Junger wrote a whole book that emphasizes shared stressors, after spending time embedded with troops manning forward outposts in Afghanistan, who regularly came under fire. The soldiers Junger observed there had little in common before being assigned their missions, but afterwards any one of them would not hesitate to fly across the country at a moment's notice to help out another, if it was needed. My core friend group I've known since I was a teenager, in scouts. I don't have much in common with them anymore, and most are cast to the four corners of the earth, but they're still my friends. My 'social network' of casual acquaintances have been drifting apart the last few years, and I don't really miss them, because shared context is the only thing we've ever really had going together in the first place. A year before COVID, at 42, I got back into rock climbing, and now that I've been on a few climbing trips with some of the guys from the gym, I feel like I'm on my way to forming the first real friendships I've made in decades; and the only thing that mattered ahead of time was their willingness to get out there and get into trouble with me.