Weird transitory friendships

It’s weird how someone can be such a complete part of your life one day and then suddenly they aren’t. It was brought home by a conversation with a very old friend of mine recently and I’ve been thinking about it. Moping you might say.
Once there was not a time when he was not included in my plans, current and future. Not for any reason like being a partner or anything. Just because he was a very good friend and it just seemed right he was part of my life.
Neither of us seem to have noticed when it was that we suddenly made plans without each other. Maybe it wasn’t sudden. Neither of us seem sure. All we know is that when we were younger we thought we would always be a part of each others lives. Now we have grown up we seem not to be.
I know friendship is transitory but sometimes I forget.