Why do borderlines sabotage their own interpersonal relationships?

Borderline. Chronic feelings of emptiness & guilt & despair with lack of trust in others, along with idealization/devaluation sense, make their relationships shaky & short-lasting as they fear rejection the closer they get to someone.
BPD. Individuals with this difficulty long for love but are incredibly sensitive to rejection and quite suspicious they will be abandoned. When they experience loss and disappointment they can easily fragment internally and become extremely angry. Best.
Not on purpose. They don't do it on purpose, really. Instead, they have difficulty maintaining an inner image of their partner still being basically "good, " during times of disappointment. During such experiences it's as if the good times never existed, and they're aware of only pain and rage. Feelings about the partner gyrate from idealization to devaluation, quickly enough to damage the relationship.