Don't underestimate how much they rely on external validation for their sense of worth.

Oh, yes. Narcissists/sociopaths are like buckets with holes in them. There is no way to fill them up with anything good, so they demand more and more and more from the people around them - more attention, more praise, more strokes, more everything.

A normal person has self-respect and self-worth and self-esteem. Sociopaths do not - it's all external, as you said - and it's one of their hallmarks.