School is the golden age for many as to making friends. It was for me, certainly.
Work becomes a natural stand in, once out of school.
I have met and made enduring friendships with neighbors. Gardeners will often bond and the setting works; being outside at home a lot; other gardening neighbors stop by and the conversation piece is the setting itself.
I live in an old condo complex with lots of grounds; lawns, streams, mature trees, pond and summer pool. It boarders a great swamp and preserve woodland, so there's lots of wildlife. Neighbors walk, fish, swim and look at wildlife here. I have made close and casual friends here in the complex.
Some of my close friends are from church.
I've heard that dog walking attracts friends and I believe it. I'm pretty sure that my daughter's dog, Crosby, has more friends than I do. When he travels up from SC he has at least 3 families that invite him (and my daughter by extension) to stay at their place in the MD/Wash DC area. When I travel south I have to stay in a motel in MD or VA.