Isn’t sad if a relationship ends so does almost every relationship created from that relationship?

There would have to be really extreme circumstances to make me stop being friends with a friend's ex if we became friends while they were a couple. My best friend's girlfriend is my other best friend. If they broke up there is approximately zero chance I'd stop talking to either.

You mean when you hop from relationship to relationship thinking it's their fault, but after a while you face the fact that it's yours?

Sometimes you have to rethink completely.

Yep