Most Helpful Guy
At the time maybe they " think " that they have reached a stagnant point in the relationship and only can lash out in various asshole ways.
Then after a while they realize they have nothing or no one and try to come crawling back.
Most Helpful Girl
Because they think they can treat you like crap and then come back like nothing happened. I've had plenty of exes do this to me.
I think sometimes people don't realize or think about the effect of their actions on another person. So they think they can do hurtful things and just come back and have things go back to the way they were.