It's not just this, but it's the frustration that comes from not being able to, for lack of a better term, fix BPD. You can pour your heart and soul into a relationship with someone who is borderline (as I did), and it will all be for nothing as soon as they split you. You can have a relationship spanning back years, maybe it's even someone you've known since you were a child, and suddenly a switch goes off in their brain and you may as well be someone that they just met yesterday.
There is no parity between Cluster Bs and "normal" people when it comes to things like friendship, loyalty, and obligation, and that has the potential to really hurt people. Some (most?) never figure out what Cluster B personality disorders are -- to them, these people are simply evil.
At best you have some success getting them to change and improve themselves, but then they just discard you and move on without so much as a second thought about you or your feelings. Unless they need you again in the future, that is.