Rabs pretty much bang on. We all go through it. You realize as it takes effort to keep in touch with your old friends who is worth your time and who just isnt. its perfectly normal as people go on to start careers, move around, find new hobbies and genuinely grow up not everyone does at the same pace and sometimes you just need to move on.
My better half always talks about how I need more friends (mostly jabbing me) but I genuinely have maybe half a dozen good friends. These half a dozen friends are all I would consider good friends though, ones I would help and vice versa no questions asked. Im perfectly content with that and shed most of the people over the years I couldnt be arsed with.