When I was in graduate school, I saw this same phenomenon a number of times. Attractive, intelligent, female students would hook up with no-account, drugged-out bums. After observing enough of these relationships, I concluded that there were two main reasons for them: 1) The female got to place herself in the role of the "rescuer", which made her feel good about herself, much like she would feel if she had taken in an abandoned puppy or kitten. 2) It put her in complete charge of the relationship. Because the male was essentially helpless and totally dependent on her for everything, he had very little say about anything. She got to call all the shots, and she liked it that way. And I'm sure that in some (maybe most) of these relationships, both factors were involved.
My caveat is that this analysis is not based on any scientific data. It is based solely on my own observations, and may not apply to your situation.