Carey Nieuwhof has a post on dealing with negative people.
While his focus is on ministry, this applies to business and entrepreneurs as well. I’ve seen firsthand how someone with a bitter, negative attitude can suck the air out of a room.
Carey is right that you have to take measures to minimize the impact of a negative person. Unfortunately, some people are determined remain in the muck. When that happens, make sure they don’t take you down too. Some pessimists just won’t rest until you are just as miserable as they are.