Is the glass half full or half empty? This is a common analogy to describe an optimist and a pessimist. 

I think it doesn’t go far enough. An optimist is someone who don’t just think the glass is half full but also hopeful that maybe one day it will be filled to the brim. 

Where as a pessimist would worry not just because it’s only half full but maybe the glass may some day break, losing everything.