I think perhaps more than any others, some of Alice's and Twisted's posts have been the most revealing in terms of what is going on in the hearts and minds of someone who suffers from HPD.
I think my HPD GF (yes fail at breaking up) revealed a few things to me as well, though very indirectly, that have slowly opened my eyes a bit more, though I am still squinting

In Alice's posts, two ideas really stood out - the need to be loved by ALL to feel valued, and the ability (in her past) to utterly block out any sense of guilt over using others if there was something she wanted from them, while pretending to be emotionally interested.
In Twisted posts, just many insights into the torment someone who is struggling with HPD goes through trying to 'act' as NONs act, but often without really feeling it.
And a big problem for us who get involved is that we're wired to respond to behavioral queues; it's hard for us to really believe (even if we know it in our heads and our intuitive alarm bells are ringing) that there is a big disconnect between the act we see, and the emotional state of the person doing the acting.
I do think those with HPD 'want' to be loved, so it's not all just an act, but having looked over my list again, there was a repeating theme (so really I was being redundant and the list could have been shorter). The repeating theme is the person who suffers from HPD can't emotionally be committed to any one person for long. Which for many NONs is the opposite of what we need.
NONs have some friends, some family, some acquaintances, and these people play various roles in our lives, but it's not (or rarely) anything sexual/romantic. We have certain feelings for these people, but a sexual/romantic relationship is something very special/different by comparison. We value it, feel it deeply, emotionally, we treat that other person as something special, not easily replaced, feelings that we might have for others if we were single are reserved primarily for our lover.
But it appears that's something the person with HPD can't really reciprocate for long. They may enjoy the emotional attention from someone wanting them in that way, but for multiple inter-twined reasons, they can't really give back the same feelings. They can act like they do because they learn what works, but fundamentally they're heart isn't in the same place that a NONs is when they are in a relationship. I guess some combination of running due to being easily hurt; having little patience for delayed gratification that invariably comes from being involved with 1 person who just can't be there 100% of the time; the fundamental need to be re-affirmed by many others eating at them and in a way, perhaps, made worse by their focus on 1 person which can lead to situations where they must choose the 1 over the many others who could provide validation too; and more.
I guess many of us hang on because we keep hoping it will be different with us, when the HPD is involved with us, it is good - very good! I keep hoping, even though intellectually I know I'm kidding myself.