It's tough as you go through your mid and late twenties, because until then, you're often in a position to make friends by default. In school, you're surrounded by people like you (same age, same geographic area, likely somewhat similar economic station) and hanging out with friends just kinda happens.

Then you graduate, people get married and start having kids and the dynamic shifts. And now, to make friends and nurture friendships, you have to put in significantly more effort--at a time when your daily energy levels are declining.

I maintain a handful of strong friendships, but they don't look much like they did fifteen years ago in my early twenties. And I'm good with that.