Just been at a psychology guest lecture funnily enough, actually about belief and magic, but also had a lot to do with optimistic v skeptic thinking.
The primary argument was that in many tests done with people who consider themselves optimists or pessimists, or natural lucky or unlucky, those who were negative in mind were restricting their ability by failing to see oppurtunity. One test was getting groups to read and count the amount of ads on a specially designed newspaper as quick as possible. There were however 3 big half page ads that told the readers "dont bother counting- there are 53 ads in this paper". Tests suggest that people who consider themselves lucky/optimistic "seen" these ads more so than the pessimists. Came down to the idea that those think in a positive way have a more open and less restrictive way of viewing life. Negativity seems to restrict this.
In poker the benefits of this are obvious. You would find many sports people at the top of their profession who live with a negative attitude.
Also optimistic people live longer:
http://www.livescience.com/health/04...ist_heart.html
An obvious retort to this question would be, why would you not strive to think optimistically?