I found an intense Q & A that helps to unravel the extremes of idealization and devaluation (love and hate) in BPD. I have quoted a part of Dr. Robert Saltzman’s answer below:
Borderline Personality Disorder is a kind of mental illness characterized by pervasive instability in self-image, in moods, in interpersonal relationships, and in behavior. To take an example of this instability in relationships, a PBD type of person might at one moment regard a friend or loved one with great admiration, affection, and love, and then suddenly, without warning, switch over to an attitude of irrational disappointment, vehement anger, or even hatred. When this switch occurs, the BPD person will accuse the friend or loved one (or the therapist) of having abandoned, mistreated, or misunderstood her, or otherwise blame the other person for having caused the sudden breakdown in idealization and affection. Often, the BPD person interprets ordinary everyday events as intentional rejections. For example, if the therapist, due to a traffic jam or household emergency, arrived late for a session, the BPD person would not believe those reasons, and would accuse the therapist of caring more for other patients, or having lost interest in her. Regardless of any apology, the BPD person would feel rejected and abandoned, and would express those feelings through anger, depressed mood, threats (“I’ll get even with you”), or even with violence.
There are various theories about the causes of borderline personality. One being the outcome of narcissistic wounds sustained in early childhood. Such wounds could occur in any of numerous ways—here’s an illustration:
Suppose, for example, that a baby began nursing, but then the mother became ill or depressed and could no longer manage the feeding schedule. In the infantile mind, the loss of the breast might feel like a rejection, and the baby might also feel that this rejection was due to a lack of self-worth. In other words, mom has stopped loving me because I am a bad child. Once that pattern of thought has become established, other random events might also be seen as adding proof to it. Later, for instance, if a sibling receives a gift that seems better than the one that our child receives, the child might feel that this is due to her lack of value.
Now, each person’s sense of self and self-worth is built up (or not built up) as the result of many thousands of such individual instances. A child who receives sufficient emotional support usually will become an adult with a workable sense of self-worth, but a child who lacks that kind of support may become an adult with a sense of self that feels not solid and stable, but full of “holes” like a Swiss cheese. Such a person, having a core self which is not solid but filled with gaps, will look to others—to relationships—to fill those gaps. That is why such a person is so super-sensitive to any aroma of rejection, for the “rejection” is seen not just as unfriendly or unkind, but perceived as a threat to the very integrity of the self which might, if not supported properly, disintegrate entirely into a kind of “insanity.” And this is a very sticky situation, because the borderline type is looking, constantly and hypervigilantly, for the kind of support that he or she lacked as a child. She or he wants, in a way, to be treated like a child: never disappointed, always cared for, always protected from the cruelties and realities of ordinary life. And such a person demands that kind of treatment from other adults. But, although some people are able to offer that kind of support to an actual child, few would be able or want to offer it to another adult. Hence, this wounded self, which wants an extraordinary level of support and consideration–an impossible level really–by its very demand that such treatment be provided, is setting itself up for rejection. Then, when the inevitable rejection comes, the ego, the “self” begins to come apart, to disintegrate, and the anger, mood swings, and all the rest follow.
All of us experience changes, losses, and rejections in life, but if we have a reasonably firm sense of self, we are able to roll with those inescapable punches, and keep going. The borderline personality type often cannot roll with any punch, no matter how trivial, and so, instead of moving on after a perceived slight, may become fixated on having been “rejected,” and then blame the person who did the “rejecting.” As you are experiencing, this blaming often damages personal relations, drastically, making it impossible for anyone save the most determined friend or therapist to continue interacting with the borderline person.
Quoted from Source (including featured image): dr-robert.com