Why is he so optimistic? It pisses me off.
Why would his optimism make you upset?
Most people I know are so negative about life, how can he be so positive and upbeat?
Being optimistic just means he sees life a little different than you, he looks for the good in things and looks for the positive outcomes. That's just his way of dealing with things and it's no reason for you to be upset.
I can't work with this.
Have you considered going to a therapist? I'm worried that your extreme reaction to his optimism is coming from a place of depression. It can be good to talk about your feelings with someone else, and this might help you deal with this guy better.