I'm a 21F, &amp; as I have been paying more and more attention outside of research &amp; just my own harrowing experiences with a person who has BPD, I notice that 94% of the time, the person with BPD has a favorite person who is unattainable, that they can't function without? Why is that and what causes that? 

The one who becomes a favorite person often ends up becoming a shell of themselves, similar to that of narc victims.