I had read all of 2010's stuff, and it was good, but it didn't sufficiently explain the intensity/depth of feeling I had for my pwBPD. And while I know I had some N tendencies in dealing with my pwBPD, I am not NPD, and on test for it come out low of normal... . which is pretty consistent with being a codependent/people pleaser... . but still didn't explain the intensity.
The fact is - you bought it all - the idealization, the mirroring, the words... . it was stronger for you than the actual logic and actions. The intensity is because temporarily that "core wound" felt healed. Once the temporary band-aid is ripped off, it is a bigger wound than what you started with.
Why? Lonely, narcissism, childhood wounds, codependency, addiction, emotionally immature - that is something for you and your therapist to figure out.
The reality is if we stayed in these relationships a significant amount of time - we overlooked logic and chose to believe the words that were said because that FELT better. It FELT better (more intense) because it was simply a numbing agent on our core wounds.