It is sad I had a best friend who I just lost out of touch with when we both entered college. I don't remember if I did something wrong or what, I remember he even lived with me at one point when he was having problems with his parents in highschool. Yeah I only have 4-5 really good friends (keep in touch with often) and then a bunch of acquaintances/work people.

Contrast to high school when I had tiers of friends/different groups. It is an exposure thing, more people in education settings.

The core loop of new friend formation is repeated casual contact.

School is naturally good at forcing this, since we are filtering in and out of different class groups.

If you want to recreate this in post-school life, you have to join high membership interest groups, like casual sports groups (kickball, tennis, golf).

Game nights at the local pub work well too, since there will be a rotating cast of characters to make friends with.

Basically, you have to seek novelty and regular re-occurance to make "natural" new friends.