It's hard. The vast majority of friendships I have formed as an adult haven't really endured. A few have, but largely they only last as long as whatever shared thing you have expires. At my last job, I made quite a few friends. Played volleyball with one person, went to the bar a few times with him and another guy we work with. Once I stopped working there, I stopped talking to most of those people. Some text messages here and there, but that is it. I have tried to keep up a bit, but ... it just always kind of fizzles. Hell, there is one person I thought I was going to be friends with for the rest of my life. But after leaving that job nearly five years ago, I have seen him once. Last text messages between us were from six months ago.

I did stay close to one of my colleagues after leaving that job, but he died ~2.5 years ago.

Most of the people I talk to regularly are people I have known for 20+ years.