Because it's usually right to be an optimist. See this great interview with Marc Andreessen from the other day:
http://nymag.com/daily/intelligencer/2014/10/marc-andreessen-in-conversation.html [And note how you can just sense the small-mindedness of Kevin Roose in how he asks some of the questions (sounds like Mr. Roose would be the guy wanting to outlaw cars just to save the stable-boy jobs)]
The pessimists do not change history or advance humanity; the optimists do. We're living in an incredible time, and, well...
If you don't believe me or don't get it, I don't have time to try to convince you, sorry.