There are a few studies out there discussing the merits of optimism, pessimism and realism. Ultimately, these studies try to conclude which one is best and often conclude that one has a higher survival rate than the others. Instead of linking to all those bogus studies, I suggest you read the following quote, listen to Styx and come to your own conclusions.
“The pessimist complains about the wind; the optimist expects it to change; the realist adjusts the sails.”
– William Arthur Ward