Just life. I’m in my 50s. People change, move, fade in and out of my life. Over the years I’ve had many acquaintances, a handful of good friends, and one best friend. Now my best friend I’ve been friends with for decades. We may only email/text/call every couple weeks/months, and at this point we don’t meet up but once or twice every year or two. When we do, it’s like no time has passed. The same stupid jokes, the same taste in entertainment, similar philosophical discussions, etc. The bond was forged through experiences we had growing up together. I can’t see developing another friend like that again because I lack the naivety, energy, and freedom (really just youth) that I once had.
I said it may not happen, because it’s a commitment. Not a conscious one. It’s, as I said before, life. Situations come up, life changes, close friends move away physically or emotionally. They may come back into your life, they may not. But having a “best” friend is something I think I took for granted a long time before noticing that a lot of people’s “best” friend is often talked about in past tense.
On the other hand, I have some friends who have a best group of friends. That is to say, a handful of people who have stuck together for decades and really work as a best friendship group. I’ve always been a little envious of that.