Optimism is a good characteristic, but if carried to an excess, it becomes foolishness. We are prone to speak of the resources of this country as inexhaustible; this is not so.
Most of us stand poised at the edge of brilliance, haunted by the knowledge of our proximity, yet still demonstrably on the wrong side of the line, our dealings with reality undermined by a range of minor yet critical psychological flaws (a little too much optimism, an unprocessed rebelliousness, a fatal impatience or sentimentality). We are like an exquisite high-speed aircraft which for lack of a tiny part is left stranded beside the runway, rendered slower than a tractor or bicycle.
History demonstrates that participants in financial markets are susceptible to waves of optimism. Excessive optimism shows the seeds of its own reversal in the form of imbalances that tend to grow over time.
Progress is Providence without God. That is, it is a theory that everything has always perpetually gone right by accident. It is a sort of atheistic optimism, based on an everlasting coincidence far more miraculous than a miracle.
The most common cause of low prices is pessimism - some times pervasive, some times specific to a company or industry. We want to do business in such an environment, not because we like pessimism but because we like the prices it produces. It's optimism that is the enemy of the rational buyer.