I suppose everything - well almost everything - has a start and finish. Friendships included. People change over time, that is a given. Not only because of experience and stage in life but also formed or reformed by their environment. Perhaps it is hard to lose a friend, I can imagine. Though I can't say I have such experience. Sure, some friends I don't see that often anymore, some very very rarely, but when I meetup with them again - this can be years of no meetups - it feels the same as always, true friendship.
some people are in our lives for a season, some for a reason and some for a lifetime
I can add to this: Close friend for a day or weekend. Experienced this in clubs and at festivals I went to frequently over the last 3 decades or so.