I spent a few years running a startup that was tackling this very issue. Sadly getting $ for "solving social isolation in America's cities" was hard to do, but

I spent a few years running a startup that was tackling this very issue. Sadly getting $ for "solving social isolation in America's cities" was hard to do, but I am still confident that the approach the company was taking would have worked for at least a double digit % of people. tl;dr friendship is formed when there is alignment of values/personality (but not in the way people always expect), and also when a lot of time it put in. The time thing is powers of 10. IIRC, 10 hours to get to know someone as an acquittance, 100 hours is a casual friend, 1000 hours is a good friend, and 10,000 hours is a lifelong friend you can really depend on. The way to fast track this process is to create a shared feeling of oppression. Military boot camps rely on this technique to rapidly form group bonds. It is the same idea behind quite a few team building outings as well. It is also why college dormmates can form long lasting relationships, lots of time spent together during a period of high stress. Same goes for people working at high pressure startups together.