What causes BPD?
Learning that there is no such thing as unconditional love during early life. Never having been loved unconditionally up to the age of 12 or so. Child sexual abuse. These are three of the things that cause BPD. I have no doubt there are many more.
If you know someone who has BPD and suffers from it — which means you suffer from it too — try to remember that the behavior is not deliberate, not intentional. The irrationality, the clinging, the very dramatic gestures — none of it is within the average BPD person’s control.
When BPD takes over, that person has entered hell and sees absolutely no exits, no way out, except fighting through, the exact same way that person has done all along in order just to survive life without unconditional love.