Optomism and pessimism are both signs of mental illness. One shouldn't expect either a fantastic or dismal outcome, you should try and predict the rational range of outcomes of any situation which you have influence over or which preparing for will benefit you. To adopt any other position is setting oneself up to less from the opertunity of foresight. It doesn't matter if you are an optimist facing negative consequences that you don't have a plan to ameliorate or a pessimist who has positioned himself so conservatively that he can't feed on the bounty that was likely his if he had just tried to accurately gauge his good things he had coming. If the matter at hand is something one has no control over and which preparation for is futile like an election or a sports ball game than pessimism is obviouslt the preferable path. A pessimist will go through life being pleasantly surprised how well things turned out or at least being unsurprised at the winds of fate while an optimist gets to spend their time either being disappointed or mearly satisfied that things have turned out as suspected. The path of greatest profit will always lie with the rationalist who tries to predict and prepare themselves for a range of probable outcomes. Anyone who wants to engage in a bout of magical thinking about changing probable outcomes with the power of their will can just eat a dick. Optomism is more than uncool, it's a sign of mental deficiency.
I tend to disagree. I think the way I define optimism (and how I tend to view myself) is that even if I fail (which happens with striking regularity) that I think that I can learn what I did wrong and do it better at the next opportunity. Maybe a better way to divide the world isn't between optimism and pessimism, but rather hope and fatalism. One of the best pieces of advice I ever got was from mk. He said, "Never marry a pessimist." I couldn't impress anything better on any unmarried person.