BPD is a developmental issue, that's passed along generationally.
Borderlines beget Borderlines, because their offspring must develop mighty defenses to protect themselves from further emotional harm. Borderlines fear attachment to lovers, spouses and even their kids. How can it feel SAFE to attach to a mother who can't welcome and return our love? It can't, and that's our blueprint for all relationships that follow, from infancy onward.
Attachment means loss of Self (what little there is) for someone with BPD features, so it feels way too risky. The scenario goes something like this: "If I let myself truly bond to you, and something happens to take you away from me, I won't survive that loss~ so I can't let you become too important to me, or I'll perish."
Real loving takes courage. When a Borderline moves closer and deeper feelings are prompted within, it scares 'em. They immediately have to pick a fight, distance, or find fault with you. This pattern never changes, except to get worse as time goes on, or as you move into fortifying the attachment between you (moving in together, getting a dog or having baby together, getting engaged, married or buying a new home, etc.).
Anytime the relationship goes through an escalation of any type, the Borderline will act-out their anxious distress over feeling closer to you, by angrily flaring up, screaming, accusing you of heinous things, etc. The better you are to them, the worse they treat you~ WHY? Because they had an impaired BPD mom who convinced 'em they were not WORTHY of being loved and cherished, from infancy onward.
YOUR BORDERLINE WILL NOT OUTGROW THESE TRAITS, WITHOUT HIGHLY SPECIFIC, SPECIALIZED HELP DESIGNED TO HEAL THEIR HEART, AND HELP THEM GROW INTO THEIR CHRONOLOGICAL EMOTIONAL AGE, RATHER THAN REACTING LIKE A TODDLER TO THE SLIGHTEST STRESSOR.