Because – hands up – someone who has been close to me for much of my life, probably has it. I’m not saying who, or even divulging their gender, but the way they are has caused huge problems for me.
The NPD character in my books is not based on this person. She is just a character who happens to suffer from the same set of symptoms, which, I have learned, develop through trauma in early childhood. NPD traits aren’t consciously chosen by the sufferer – and yes, they do suffer, in my experience – often far more than their victims, because the trauma has rendered them incapable of useful introspection, so they can never improve anything for themselves and are therefore stuck in their behaviour and response cycles.
The people around them can escape from the cycles, or at least learn to adapt and maybe understand them. The person with NPD cannot. My understanding (which may be faulty: I am not a doctor) is that there is no better life for them, ever, unless they receive very effective trauma therapy and for that to happen, they would have to admit to the nature of their problem, which, with no introspection, is pretty much impossible. Nowadays, I feel very sorry for them.
I’ve witnessed these symptoms at close quarters for a long time, so I do know something about how they manifest, but none of my book character’s actions are directly based on that experience. The book character and the person in my life are two, very different people.