I think the measure of a friendship isn't how you handle the 80% of life that ranges from "pretty good" to "pretty shitty". That stuff tends to be fairly easy to deal with--you offer congratulations or a shoulder to cry on and it's all good. The tough part is that other 20% that's ecstatic or tragic--think about your best couple of friends and how they would handle you being in one of those states, then compare that to someone you think of as an acquaintance. Your really good friends know what makes you tick when important stuff happens. Now, the real test of friendship's quality is what happens when one of you is an ass. Every relationship worth the name has a certain amount of scar tissue--times when you've hurt the other person, and vice versa.
So why do they survive? It's because you also have this bank of goodwill that you've built up over months or years. "Yeah, Jason was a real jackass, and I want to kick him in the head. But you know, he's put up with my shit before, so..." Really good romantic relationships, I think, have a strong friendship as well. But we have this tendency to think that the romantic element is sort of this separate thing from the friendship.
Anyway, so why do you break up with someone? All sorts of reasons--breaking trust, loss of momentum, fighting, money problems, blah blah blah. In the end, though, it comes down to one thing, and that's that your bank of goodwill isn't enough to outweigh the shit you're putting up with. (Mind you, that's most of the time--there are some instances where it just sort of fizzles, and goes back to a platonic friendship. But those are boring breakups that your friends don't want to hear about, so we're ignoring those.)
With your average breakup, you've got some baggage associated with it, right? In the end, then, when you try to go from ending a romantic relationship back to a "good" friendship, you've got a problem. The goodwill isn't there anymore. So what happens when the other person is an ass? "Jason is such a fucker! This is JUST like what he did when we were together!! God, I can't believe I EVER PUT UP WITH HIM!!!" You've used up all the freebies already, and now you're back to a point where you have to develop the friendship, in some ways, from scratch.
--jhf 10 May 2003