Love is a highly sustainable emotion, except if you're a Borderline. We call people with BPD mercurial, because they're incapable of sustaining emotional constancy. This issue is central to their come-here/go-away cycles of behavior.
Just like a toddler throwing a temper tantrum, if you try to mollify their mood with a cookie or lollipop, it almost instantly shifts to being happy and agreeable. The same phenomenon exists for underdeveloped adults~ they'll love you one minute, and hate you the next.
Borderlines suffered attachment difficulties during the first days and weeks of their life outside their mother's womb. Due to a lack of emotional attunement and consistently nourishing bonding with Mother, they grew up without the ability to trust another with their care.
Borderlines hungrily SEEK the nourishing primal/infancy experience of a safe, secure attachment they missed out on during infancy with their mom, but cannot find it. Their template or blueprint for attachment is distorted by having had to experience Pain of Yearning comingled with loving, since they were newborns.
Thus, someone who's nurturing, caring and responsive to their emotional needs doesn't fit their paradigm of what "true love" is supposed to feel like, because he/she doesn't inspire the dramatic sensations they felt as young children, in the absence of a secure maternal bond. For the Borderline, Love = Pain, and vice-versa.
Borderlines are addicted to emotional intensity. If there's no adrenaline rush, "it can't be real love." If their relationship doesn't offer intensity in the way of strife, conflict and chaos, they feel flat, empty and bored. These uncomfortable feelings are projected onto their lovers as being at fault for prompting them, when in reality, calmness and serenity force the Borderline to experience their CORE VOID.
The core void (which I thoroughly address in my book, DO YOU LOVE TO BE NEEED, OR NEED TO BE LOVED?) ONLY exists in those who've dissociated from a litany of emotions since they were toddlers. Dissociation from emotions leaves one feeling empty, flat and dead in a region of their body where all sorts of feelings and senses SHOULD reside, but were killed off and buried since they were tiny, in order to survive in their home environment.
The business of repairing a Borderline involves re-associating them with a plethora of feelings they've grown to fear, dread and divorce themselves from, when they were too young to endure em. Self-worth exercises are also a crucial part of getting a Borderline well. The client must consciously work to replace the negative, mental self-flagellating tape that runs 24/7 in their psyche, so that self-worth repair is possible.
In my nearly 3 decades of helping people with BPD learn how to repair themselves, I have not discovered any other way to help them outgrow their borderline personality disorder traits, and create a life for themselves that feels happy, content and richly satisfying~ whether they are coupled or not.
Behavioral and psychotherapeutic techniques fail someone with BPD, because they're small children in adult bodies. You wouldn't clinically try to get a 3 or 4 year old to analyze their pain, because it's well beyond their capacity to benefit from it.
The same holds true for Borderlines. BPD recovery depends solely on emotional development and self-worth repair. Any insight-based modality of treatment simply reinforces their life-long habit of attaching meaning and reasons to their pain, but never resolving and healing it.