Ah, I get ya. In my experience, it's been hard to either keep myself from being wistful about what was or keeping him from trying to re- ignite the flame. Been on both sides of that, and with someone I spit on friendly terms with, it was always uncomfortable to the max.
I also think a lot of it has to do with how your relationships tend to develop. I've always just "ended up" in relationships with people I was already friends with, so going back to that once the relationship has played itself out is probably easier than establishing an entirely new sort of relationship...