Who do people insist on going back to someone (relationship), who at one time treated them poorly?

Say, if someone was put through hell while in a relationship; has broken up with that person, then decided to get back together and then, guess what, the person treats them poorly again. So, they decide to break it off, take a year or so off, then decide that they`re ready to date again and they find someone who treats them like gold. They date them for long while. They tell the person that they love them and when one little thing goes wrong, they dump this good person`s ass and decide that they `never stopped loving their (crazy – my words) ex` and they`re really sorry for hurting the good person, but they`re still in love with the ex`...

The ex claims that she is reformed (no longer an evil person) and, so, one has to jump back into their arms as soon as a little thing goes wrong in the new (good) relationship. Is this person seriously cruel and fucked up for leading partner number 2 on, when she later claims that her feelings for her ex had never changed..

Messed up, huh..

Bitter..yup!