Every place I've left, the best I could muster with a straight face was a bland tautological platitude — something like, "Oh, you know, just going to do something different."
I don't exactly have tons of experience quitting jobs or ending relationships, but I've never understood the relentless navel-gazing of "why" that seems to come along with most people quitting their jobs. And, besides, what's the point of expounding on how things could be better (in an "exit interview" or otherwise), when everyone knows all that feedback will have no impact?
It seems to me the practical part of the conversation is simply, "I'm ending our relationship." And then — everyone moves forward from there.