It's hard. My wife and I talk about this often. Having kids can help because you kinda end up spending time with the other kids' parents. But I have to admit that this actually hasn't worked too well for us: haven't made any friends this way.

It's doubly complicated because life is so mediated through the internet now. Most of the new relationships I've formed in the last few years have started online -- due to bitcoin mostly.

Most practical advice I have? Go to meetups about topics you're interested in. Feels awkward at first, but it's a great way to get to know people.