Some people say that this impairs right-brain development and the capacity to self-regulate affect. The upshot is that the BPD has no self or object constancy.
All of us are sensitive to being empathized with, being "seen" and understood. So, someone who is good at mirroring other people's states can sure get their attention. Mirroring can happen through how a person looks in your eyes, is attuned to your actions and postures, interests and dreams. it can happen on many levels.
Right now I find myself stuck on the idea that what was being mirrored back to me was my best projected self - all the bits I like about myself and how I am in the world. The 'bad' parts were invisible to him, and to me. This was intoxicating. However, it feels as if what I actually fell so hard for was really just myself! I fell in love with an idealised self, not another real person. So I am just as culpable, just as sad and partial a human being as he.
There's more to it than that, sure, but on the whole I feel as if I am not fully real and being in that relationship allowed me to see how much my being in the world is an illusion. I fell for an illusion of myself. I don't know how long the process of figuring this out is going to be.
But the BPD has no constant sense of self or other and so is reliant on mirroring someone else (being intensely infatuated) but it isn't stable. The BPD had such negative experiences with the mother figure that the BPD starts feeling engulfed (no boundaries) and then starts raging, splits, and abandons the person he/she was infatuated with to avoid his/her own abandonment fears. Then the BPD starts the whole thing up with someone new and the cycle goes on. Mirroring happens when healthy people fall in love too, but healthy people have a sense of self and object constancy and so intense infatuation is just part of a relationship--not the whole thing. I think one of the things that makes us vulnerable to BPDs is their ability to capture this intensity that was surely a part of our very early preverbal development. I think we are vulnerable to mirroring.
Are we less vulnerable if we have a more complete sense of ourselves and are more authentically and fully in the world? Accepting of our dark sides and our boring bits?