I think every person picks someone because of the qualities they see. Some adjust to others because they do not want to be lonely. They would rather put up with abuse; loosing oneself while giving permission for the other person to fill their needs, desires, loneliness, identity and worth. When this happens the abuser sees the perfect opportunity to be your all and in all, but not in a good way. They get to hold the key to your heart rather than you holding your own key and guarding it with your life.
Why is it so hard to let go? Possibly because that person filled a void that may have been there already. This may not be the case but if you are a kind and loving human being you will be the target of a narcissist.
As an analogy what if you saw a dog owner beating their dog, attacking their dog verbally, calling their dog names, what would you think?
Or let's say you saw a parent abusing a 3 year old child, beating the child, calling the child obscene names then afterwards telling the child let's go to the toy store. The child gladly forgets the abuse and is overjoyed thinking they are going to get a toy. Then while walking through the isles the parent abruptly says, "You can't have any of these toys because you are a rotten kid. You don't even know how to tie your shoe laces, you are stupid and nobody wants to be your friend. You don't deserve anything and when we get home I'm going to punish you with no dinner. I'm going to call your teacher, your friends, your grandparents, tell them all how you gave me such a hard time today and I don't know what to do with you." They get home and the parent does exactly what they said they would do. At the end of the evening, the parent kisses the child on the forehead and says I have a wonderful treat for you tomorrow. We are going to the ice cream shop. The child gets all excited and thinks, see my parent loves me and forgets the toy store incident. Once they get to the ice cream shop the abuse and torment will start all over again. The child takes one bite of the ice cream while the parent grabs that cone, throws it into the garbage and begins telling the child how fat they are. " You can't have this ice cream and when we get home you will be sorry you took one bite of that cone. (Crazy making/word salad)
So take this all into the adult world of the narcissist's manipulations.. Hoovering is "Let's go to the jewelry store, let's go to the coffee shop, let's go get dinner." If you know what is coming, then why go? When you think about it... the whole relationship with a narc is about discard. They need to pound you like mince meat before they have their grand finale and once they do, hoovering is a way of having an encore performance for personal enjoyment and entertainment. They live for fireworks which explode in final glory. They are proud of their exploit to rip you off of your self worth and dignity. Are they hoovering because they care? No, they just want another round of fireworks or TNT whichever works for them.