Fiction writers tend to be pretty negative people as well. I think writing and programming have one very important thing in common: most people can ignore the ugly parts of humanity. Writers and programmers spend years staring into it. Writers do so to better understand people and write believable characters. Programmers do so to better understand people's behaviors so we can solve their problems.

I work in social games. Nothing makes me more negative than running a user test and finding out that treating your users as if they were mentally deficient 4 year olds results in more people coming back to play again and more people willing to give us their money.