The thread on accepting abuse reminded me of a theory on abuse. I've found it really helpful and it seems to strike a chord with a lot of people.
Psychology can sometimes be more of an art than a science, so keep in mind this is just one theory, sort of a way of framing a perspective only if it makes sense to you...
The theory is that people who continually engage in abusive relationships, either with family, friends, or romantic partners are unconsciously trying to reenact an early abusive experience and change the outcome.
As in, I was hurt and I want a do-over to prove that I'm worthy of love and respect and/or that I'm so powerful I can change someone who would otherwise hurt me and cause them to love me instead. Thereby diminishing the impact of the early abuse because, if this time is different, then that one was just a fluke, and it isn't me.
The problem is of course we can't change other people's behavior and engaging with abusive people only gets us abused more. But, when we didn't realize why we were in the relationship in the first place, we confuse being attached to our deeper issues with being attached to the deeply disturbed person we are in a relationship with! You might not really want him/her/them (including "them" for people like me who had simultaneous abusive relationships with friends, family, and spouse) What you might really want is relief from the original pain. And, without realizing it, you're clinging to the hope that this relationship is the way to achieve that.
The same thing happens in the reverse where people who are abused become abusers, wanting to reenact the scene(s) and emerge on the other side as the "winner", so to speak, instead of the "loser"...this time.
The way to break out of this, according to the theory, is to really process and make positive meaning out of the original abuse. It wasn't my fault, I didn't deserve it, he/she is unhealthy and took it out on me, now I'm stronger and more compassionate (or fill in the blank with your own meaning) and I can let go and move on, leaving the past exactly where it happened and not giving it any place in my present or future. This person and this relationship does not represent my salvation or my relief from the pain, I do! It's in me. They're actually making it worse.