You hear narcissists talk about how people look

...how much they weigh, if their clothes or hairstyles are acceptable, and on and on.

How things "look" is important to them.

They do comparison thinking while looking at others to see if they measure up or not, and if they themselves are bigger or better. If they can take you down a notch, they'll feel larger and more important.

Jealousy is a major factor

They tend to either think that people are jealous of them, or else they are jealous of others. This thinking is based on external validation for themselves rather than internal validation. Because they lack a solid sense of self, their world is all external, based on how they appear to others.

-excerpted from 8 Tips to Protect Yourself from a Narcissist