There's a really strong tendency to agonise over "why?" and "how could they do that?" I find it helps to remember that in the olden days, such people were simply called mad 
I'm olden enough to remember when people talked about someone having a god complex, being an egomaniac or megalomaniac. In some ways we were a bit smarter before we had all these fine-tuned diagnoses to toss around! If someone was known to have a 'god complex', nobody bothered trying to change them or to make them see. You just pandered to them as necessary and made sure you didn't give anything away.
I also find it helpful to remember the definition of a personality disorder is an extremely rigid personality. We are all narcissists, psychopaths, histrionics and the rest: the difference is that it's fluid for us. We segue between states, recognise that other people do the same, and can change mode as needed. People with PDs are stuck forever in one or two of these states.
Think about some beautiful narcissistic moments of your own. I go back to my teens, when I'd spend five hours getting ready for a night out. After staring at my reflection like it was a canvas, airbrushing my face and hair to near-perfection, I'd go out and expect people to look at me. Seeing admiration on their faces, receiving and exchanging compliments, feeling like I was in my personal spotlight and stage-ready: it was the main point of the night out. All of us were polished-up egos on lanky legs, shining those lights on ourselves and each other. Saturday nights, we were glamorous & sophisticated, beautiful & accomplished, witty & adorable. Being ignored, criticised or found uninteresting would have been awful. We made the effort and ego strokes were our reward.
Perhaps you weren't the vain, nightclubbing type of teenager but you will have had your moments; perhaps on your wedding day or the graduation ball. Massive effort & painstaking care, rewarded by some hours basking in the flattering glow of reflected admiration.
For a narcissist, all of life is like that. It is their only mode.
If you can only exist as a reflection in the eyes of others - you cannot bear to find a charmless reflection or, worse, no reflection - then the airbrushing is a matter of survival. You must perfect your image so that, when people reflect you, it will be satisfactory and you'll be reassured for a moment that you exist and you're okay. You can't have them reflecting a useless husband or a selfish mother: airbrush that out!
And, of course, you never see the people behind the eyes you look in: you only see your own reflection.
I do feel compassion for such people, though it's taken a long time. I'm also very discombobulated at the moment as my mother's spent the week airbrushing out all the hard work I did to reach an understanding with her about my childhood. One effect is that it's knocked my own narcissism off what I consider a healthy level; it's going to be a bit of a struggle to put it back. Hence this possibly weird post, sorry. It's helping me get things back into perspective.