Optimism is an important and most of the time required characteristics in entrepreneurs.

Optimistic individuals play a disproportionate role in shaping our lives. Their decisions make a difference; they are the inventors, the entrepreneurs, the political and military leaders—not average people. They got to where they are by seeking challenges and taking risks.

[Entrepreneurs often] make decisions based on delusional optimism rather than on a rational weighting of gains, losses, and probabilities. They overestimate benefits and underestimate costs. They spin scenarios of success while overlooking the potential for mistakes and miscalculations. As a result, they pursue initiatives that are unlikely to come in on budget or on time or to deliver the expected returns— or even to be completed.

– Daniel Kahnman

But not everyone can be an optimistic individual.

the blessings of optimism are offered only to individuals who are only mildly biased and who are able to “accentuate the positive” without losing track of reality.

Now leave out Charlie Munger, who comes up with the best quotes in his usual style

Never underestimate an man who overestimates himself… – Charlie Munger

Benjamin Graham had always believed more on numbers though

Buy not on optimism, but on arithmetic. – Benjamin Graham

Be realistic when required!