For instance, a few months ago I came across the address of the parents of a girl who'd been my first real friend. Back when we were ten and eleven, we did everything together, in and out of school. I still remember the fun we had playing Pong on her old Atari (man, does that date me), the giggly sleepovers, running down to the corner store together and coming back loaded with sugary junk. So I wrote a note to her care of her parents' address, just reminding her of who I was, giving her a brief update on the last few years of my life, and letting her know that I'd love to correspond with her again and find out what's been happening with her.
Well, she wrote back in e-mail, a brief and not very illuminatory note -- oh, it was friendly enough in a polite sort of way, but obviously lacking in enthusiasm. And although I replied to her e-mail, she has never written back again. So... I can only conclude that our friendship was a part of her life she wants to leave in the past, and that all those memories we shared in childhood just didn't mean as much to her as they do to me.
The same thing, more or less, happened with the girl who was my best friend in early high school. I find her address, get all excited, send her an update and ask her about herself, she writes back with the bare minimum, and after that she doesn't respond again. I could understand if there had been some unpleasantness between us that had driven us apart, or if I had done or said something weird in my letter to put her off, but there wasn't and I didn't. All that happened, in this case and in the other, is that the friend moved away and ten or fifteen years went by. I guess, for some people, the passage of time is all that's needed to dissolve a friendship.
I suppose maybe both these friends were put off by the news that I was married with children. They're both single, and maybe they felt that with such a difference in our lives, we couldn't possibly have anything to say to each other any more. But when I was single myself, and two of my good friends married each other, it never even occurred to me to think that our friendship was over, or even that it had to diminish. Why should it? They were the same people, after all. And even when Jamie and Darla had their first baby, that never came between us, even though I was still aggressively uninterested in children myself. We still had lots of things to talk about, and still enjoyed our visits together. Even though we were (and are) living 500 km apart, the friendship continued.
But I guess it isn't that way for everybody. Either I am unusually loyal to my friends, or some of my friends are unusually cavalier about friendship, or... something.