Here's a thought...

If you stay optimistic, you'll see more opportunities coming your way since you pay more attention to its benefits than its negative effects. With that mentality, you'll give an idea a shot. You may succeed or you may fail but you will give it a try regardless... thus giving you a chance to progress. If you don't progress then you're more likely to learn from failure.

If you stay pessimistic however, you'll be paying more attention to why you shouldn't do it. Thus, limiting you to opportunities since you're more likely to already reject an idea without looking at its benefits OR you're probably too hung up on the negative effects that its the only thing you see. Being scared of failure maybe? So then being pessimistic will at the very most, provide you a very minimal chance to progress since you will be trying less ideas than an optimist would.

Curious... What would be an antonym for realist? is it visionary? Are the poor more pessimistic or optimistic? How about the rich? Are the losers more pessimistic or are they just very unlucky? How about the winners and the successful?