Why when you treat them like crap they keep going back?

Like all my exes, and im a GIRL, but when you say I DONT WANT YOU,!!!! they dont get it, i cheated on my ex bf and he wants me back but I KEEP trying to tell him that there is a better girl out there for him but he keeps saying he just likes me and my current bf wanted to get exclusive and i ddnt want that so I broke up with him now he apologizes like it was his fault. im only 19 btw and a virgin but guys just keep coming at me like swarms of fleas.