I have all my life attracted them into my life. I still trying to understand what in me makes us a vibrational match. I think family upbringing. 

They are incapable of love, which makes me feel compassion towards them, since love is the best feeling and they are "handicapped", from it.

Purrs, they are just a little toddler, like your little one, that could not "separate" from mummy. They have the emotional maturity of a toddler between 2 and 4, always in flight fight mode. Survival mode. Always throwing tantrums. There is no love, only shame, like Nico said. They are so fragile they can't even accept a constructive critic. They will react and punish you, for anything, especially when you are the most loving. They can't understand how is possible someone to love what is unlovable. They don't interact with real world like we do. All is an extension of self, a self that did not develop .It's all confabulations and Illusions. Everything happens only in the mind.
A relationship with an NPD is just a shared fantasy, inside a mirror room. They project...you, into you.
Plus, they have cold empathy which allows them to "read" people like no one. They know which buttons to press just to see you break. It gives them a high. They are so insecure, they need to put others down to feel better in their skin. They don't love themselves, they never received love, by neglect at that young age. I think, somehow, their personality is built  on this permiss that they are unworthy of love.