I've been thinking and reading a lot and I thought I'd share what I understand to be true about these situations.
The emotions that a pwBPD feels aren't like what we experience. We assume that they feel like we do. They don't. This is important to remember. The closest some of us come is likely to when some of us are discarded. That pain of being abandoned, it's something that is intense and powerful, and it leads us to some dark actions. We don't respect boundaries any more. We spy. We stalk, even if it's electronic stalking. There's that element of "how can I live without them." Now, imagine swinging to that sort of emotion several times a day every day of your life. Maybe dozens of times, maybe more. Equally, then there's the swing towards engulfment, which is just as frightening as it's basically a fear of us stamping out their free will. The "relief" from these swings are disassociate episodes, where all sorts of unpleasant sensations and thoughts can occur.
When an infant is dealing with their mother, it's a life or death struggle. They have to attach in some way because mother is life. Since pwBPD are stuck in this mode, they attach to us in this desperate way.
Think about this. They are doing everything as if it were life or death, that's what their limbic system is telling them is going on. They will die if they are alone. They will die if they get too close. They will die if they are abandoned.
When they "hook" us, they are reaching out for us like a baby needs food. They do anything they can, they give us their bodies, they give us their thoughts, they give us everything they have so they can be safe.
When they look at us with that idealization, it's like we are being worshiped, like some mad god that could strike them down if they don't please us enough. Adoration and sacrifice is what they give us. They believe that this time, this is it, this is the one.
When they cling, when push and pull is going on, the fear is everywhere. We will destroy their individuality, which is death. We will leave them, which is death. Then at some point, this fear gets to be too much, and so they have to try again. They need someone else to be safe again. If they don't they die.
It's sincere. It's not a con. They don't win anything. They are trying not to die.
And yes, we need to look at ourselves. We either loved the worship too much, or we wanted to save them too much, or we suffered through things hoping it will all change if we gave enough love, or we were lonely and introverted, or any number of other reasons. There's a reason we felt like a key in a lock, and it's not a good reason.
Otherwise, we would be that person who saw something odd and just left it alone