I've been there at least once or twice. I cared for their needs more than my own, and I could always rationalize their behavior--not denying in a "this person didn't mean it" sort of way. Rather, it was a misplaced compassion, a thought process along the lines of "they've had a hard life, and(/or), dammit, everyone deserves forgiveness, and everyone deserves friends."
For me, it was a responsibility issue--I took responsibility for their well-being.
I guess there are many reasons that one would fall victim to manipulative behavior. The manipulative people will travel the path of least resistance. Once you stand up, they move on to someone else.
And even then, the fact that someone else is invariably going to be a victim is pretty tragic.
When one has been dealt major blows by a few people, it's natural and completely okay to have a guard up as one approaches new friendships. Finding quality people is tough, and it's easy to lose oneself in the process.