1) Pity: (The nice guy routine) They feel sorry for the partner so they choose to accept them back for the sole purpose of making them feel better.
2) Titty: Probably one of the only good things that could come out of this pity filled relationship, is the titty, cuz the nice person gets some.
Thing is that after this happens you're basically stuck in the pity loop which means that eventually they will just realize that they dont really like each other and its just out of pity, and so (read beginning of this paragraph)
ps. its called a loop for a reason.