You have to experience the relationship to understand.
They are usually VERY appealing at first as they hide their symptoms for a short time. They shower a man with validation, making him feel like a king, and then little bizarre comments start leaking out, followed by bizarre behavior. Since the man is already hooked on her at his very core, he has no choice but to go for a ride that he simply can't get off of.
She seems to get "better" on occasion, so time after time the man will accept her back. But soon after, the insanity starts again. This insanity is so bizarre in nature that the man is left standing dumbfounded, not knowing what to say in this foreign, ethereal world of hers where logic doesn't exist. It literally feels like she is sucking the very life force out of a man. Whether she is present with him or not at any given time, he is bleeding, and that's no hyperbole.
It really is like a constant bleeding, and only a man who has gone through it can even imagine what it's actually like. Again, the severity of it comes from the way that she reached into his innermost core and latched into him. I'm not saying it's supernatural, but it truly does start to feel like it is because the man can't even remotely understand what's going on for a long time.
To know a BPD is one thing, but to be emotionally involved with one is an experience that can't be described. It is a fearsome thing to become aware of acute insanity in someone you care about, to have no ability to help her, and to realize that the only way to survive is to get away, knowing that she will probably threaten to kill herself. After that comes a deep sorrow, feeling sorry for this person who is so lost and beyond help. As far as I know there is no cure for BPD.