Great Question P! Here is a great article explaining one theory:
http://www.afterpsychotherapy.com/at...-relationship/

In my reading and experiences, I think that like the human body has a central nervous system, a peripheral nervous system, and many other systems; that it also has an attachment system. I'm may not be choosing the right words as I'm not a psychologist, but I think that this attachment system is a part of the developmental milestones (trust vs. mistrust) and that in order for a person to feel whole, the attachment system has to be exposed to healthy boundaries, not enmeshing, not rigid, but healthy malleable boundaries. Now, I also think that something can go wrong if the child is extra sensitive, which would make the child's view of boundaries more extreme. So, I think in therapy, the goal is to be exposed to healthy boundaries. I believe that if one is not whole and the attachment system is not 'set' correctly, that one will experience the compulsion to attach to people in order to correct the system. Unfortunately, when one compulsively tries to attach (or I guess some avoid attachment) and isn't whole that it will cause ruptures in all or most relationships.
I think the most important job of a therapist is to accept the person as is (while using healthy boundaries (not retaliatory)). If the therapist does treats the person's attachment issues (neediness, say) as if they are a sickness or disgusting, then the shame that the person already owns will be compounded. That's my two cents