I wonder if we still project our baggage on someone new, and they do the same to us. Subconsciously.
In a way it's easier to deal with your old SO (generally speaking), when you treat each other for what you are, not subconsciously expecting the same treatment as of your ex's.
I don't know if I explain it right. For example, someone new might have had their ex cheated on him, so he might subconsciously treat you as that's what you would do.
I think it all boils down to how good a person is at letting go. Like if you (general you) cannot let go of baggage in your current R, you will continue carrying it into a new one. It doesn't mean you cannot break up and let go.