Sometimes, break ups happen, and contrary to popular belief, good couples can break up (needs more commas.) When you break up with someone that you had a good relationship with, it's possible that you still love them for who they are. It's not like you suddenly hate them and everything they stand for just because the two of you didn't work out. People kinda have trouble understanding this sometimes, but it happens for some people.
However, this isn't really polyamory unless you want it to be. Even still, I can believe in polyamory. I can even believe in a polyamorous relationship, though I doubt I could ever properly function in one myself.