The root of the word narcotics and narcissism is the same: narcosis, or numbness.
An interesting way to think about this is a closed-loop system: numbness is avoidance of external stimuli, an obliteration of authentic engagement.
The narcissist, in this way, is akin to the drug addict: by focusing only internally (your wants, desires, and self-image), it is not that you do not encounter pain, but for a moment, it is all about your pain, and you numb yourself to the travails or needs of other people. This internal focus is an escape from the world into yourself: a numbing of the external mirror that improperly reflects, a return to the internal logic of escapist fantasy.