Why do we hang onto those who reject us emotionally despite knowing better?

There are so many fish in the proverbial sea – yet we hang onto the losers, creeps and those who reject us – clinging on for dear life – while they suck the living spirit out of us. Why why why?

The Psych department here is welcome to comment, of course, but laypeople could also tell a personal story if they so choose. Oh, I got some good ones… you first.