People who seek out the vulnerable are often predatory, as @JilltheTooth described. My grandmother’s sister was one such unlucky individual. When her fiance a man whom she loved with all her heart and loved her just the same died, she broke and that was enough for the awful, abusive man whom none of us can bring ourselves our to name even his children do not acknowledge him as their father to swoop in and marry her. My opinion of these individuals is low, to say the least, and I try to warn my friends when I see things going that way. it never seems to work.
On the other hand, I am one of those people who attracts damaged and broken individuals both romantically and platonically. Most of the time, this isn’t so bad. In fact, it is the most wonderful feeling to see these people pull themselves up and “fix” themselves when they remove themselves from a bad situation such as the girl I mentioned from this thread. However, it can also be very bad because if, for some reason, these people leave my life or the nature of our relationship changes, that person can break further, such as the case with one of my ex-boyfriends. I later realized he was using me as a substitute mother-figure because his own mother was, well, not a mother so much as an egg donor who allowed him to live in her house so when I broke up with him, it was almost as if his own mother abandoned him. Apparently, he hasn’t had a relationship since and has become a bit more desperate and clingy with his current friendships.