Why do lonely, needy people repeatedly fall for independent, emotionally unavailable people?

Just had to break it off with a woman I'd been seeing. She says everybody leaves her. She asked "What did I do wrong?" She didn't do anything wrong. Why would she even ask that except to make herself feel bad? I want her to move on. She's really looking for somebody to complete her and I'm like I just wanna mess around. This isn't the first time this has happened and every time I don't know how to end it without making the other person feel inadequate. I'm just not looking for commitment. I thought I was honest about that from the start, but she was the one who was really pursuing me. Why she keep going after people who can't give her what she needs? Why do I keep attracting earnest and devoted lovers when I can't return their feelings? It seems like this is something you hear about a lot. Does anybody have any insight?