When you satisfy a Borderline's painful longing for "love" from you, their craving for ya evaporates! They literally feel like they've "fallen out of love."
Most laymen and clinicians are unaware of what makes Borderlines tick. They buy into the surface shit ya can read in a diagnostic manual, which tells 'em nothing about the real internal structure within Borderlines that underscores ALL their behavior patterns when they're involved with someone.
This is why psychotherapy typically fails these people. It does not address the core trauma wounds at the heart of their self-protective defenses. No amount of intellectual understanding that might be gained through years or even decades of psychodynamic intervention, dismantles attachment fears in Borderlines.
Only esteem-building work and the acquired confidence that comes from learning to endure any and all emotions in their body and RESIST the compulsion to attach meaning and reasons to those sensations (hyper-analyze 'em), builds enough core and ego strength to confidently share their Authentic Self with another, and NOT fear succumbing to losing oneself in the process!