Borderline personality disorder is a cluster b personality disorder. It's in the same cluster as narcissistic PD. It shares some traits there is often over lap. Additionally there are often both PDs represented in a family system.
Borderline personality folks if I understand correctly often have issues related to abandonment and or attachment. They are very attention seeking and can have narcissistic tendencies.
There tends to be emotional liability with BPD folks. Ther emotional state can change in a second. They often have very black and white, on or off, all or nothing thinking. They often play push pull. This is because in part they cannot tolerate any shame or self acknowledgement of being wrong.
Then tend to get triggered by real or interpreted events, actions or statements. This often accompanies a sudden mood change or sudden change in how they rate someone. c A person can rapidly change from having a positive status with the BPD person to being looked down on.
People with BPD often relate to the world with a "feelings are facts" approach. Instead of looking at the details and facts of what they encounter in life, they filter everything through their current emotions. These feelings are facts to them.
Because they can change emotional states so rapidly, they act like nothing happens Ed whole the other party is still licking wounds and recovering emotionally from difficult interactions with the BPD person