My guesses:

A rubbish sub, like a rubbish dom, does not like herself, or himself. She isn't there to be tamed or owned, she's there to hand over her self-esteem, what there is left of it, to someone else.

Some signs of this may include a previous pattern of always choosing control-freak psycho Doms, or a total lack of will or decision power in life.

Dominating someone without a will must be ultimately as satisfying as beating up a cripple.