A psychodynamic model (Winnicott, Masterson): the narcissist builds a polished "false self" to win admiration because the "true self" underneath feels empty or unbearable. The mask isn't a costume removed at home — it's load-bearing.
The charming, generous person the world sees runs full-time in public. What you live with at home is closer to what's underneath — and the gap between the two is the whole story.