Whenever I heard someone bloviate about the power of positive thinking, that "just being happy" was all that you needed, I rolled my eyes and thought 'here we go, another instance of mindless platitudes making people think they know the deeper mechanics of human psychology'.
However, there appears to be sufficient research to suggest that merely being optimistic, even when there is no reason to be, should be encouraged in life, as this article reads:
"The study found that women who were optimistic had a significantly reduced risk of dying from several major causes of death — including cancer, heart disease, stroke, respiratory disease, and infection — over an eight-year period, compared with women who were less optimistic."
...
"The most optimistic women (the top quartile) had a nearly 30 percent lower risk of dying from any of the diseases analyzed in the study compared with the least optimistic (the bottom quartile), the study found. The most optimistic women had a 16 percent lower risk of dying from cancer; 38 percent lower risk of dying from heart disease; 39 percent lower risk of dying from stroke; 38 percent lower risk of dying from respiratory disease; and 52 percent lower risk of dying from infection."
It is conceivable that the optimistic women are the ones who are not experiencing life's hardships, hence why they are optimistic. It is also possible that this is a matter of correlation, rather than any significant causation.
Nonetheless, it is nothing short of fascinating that being a mindless, smiling drone, unable to ponder the complex, often terrifying facets of life, has a better chance of survival than a well-educated realist.