An excerpt from The Power of Negative Thinking:
Ancient philosophers and spiritual teachers understood the need to balance the positive with the negative, optimism with pessimism, a striving for success and security with an openness to failure and uncertainty. The Stoics recommended “the premeditation of evils,” or deliberately visualizing the worst-case scenario. This tends to reduce anxiety about the future: when you soberly picture how badly things could go in reality, you usually conclude that you could cope. Besides, they noted, imagining that you might lose the relationships and possessions you currently enjoy increases your gratitude for having them now. Positive thinking, by contrast, always leans into the future, ignoring present pleasures.
The social critic Barbara Ehrenreich has persuasively argued that the all-positive approach, with its rejection of the possibility of failure, helped bring on our present financial crises. The psychological evidence, backed by ancient wisdom, certainly suggests that it is not the recipe for success that it purports to be.