The NYTimes ran an article February 28 by Jonah Lehrer regarding the "upside" to depression.
In some ways this represents a recent trend in articles and books challenging the Positive Psychology approach to the possibility and desirability of seeking and achieving happiness.
The "upside" approach emphasizes the evolutionary benefit to individuals and societies who worry about their lives and can anticipate potentially fatal consequences.
As I see it the existence of negative biologic and psychological traits in the human gene pool reflects the overall benefit to the human population when seen within a context of balance and moderation.
For instance, there is the well known gene which produces the dreaded disease of sickle cell anemia when in the homozygote state (individual has received two genes, one from each parent). When present in the heterozygote state (individual has one gene and is asymptomatic) the individual is relatively resistant to malaria. Statistically, within a population, far more individuals will be heterozygote and therefore malaria-resistant. In a sense any one individual may suffer for the benefit of the population as a whole.
There are many other congenital disease of genetic origin which are being detected in individuals which may allow for some genetic engineering–couples who possess potentially lethal or disease producing genes may elect to adopt or even consider therapeutic abortion.
Psychological predispositions could be understood in the same evolutionary sense. Some degree of worry can clearly save individuals and groups when preparations for potential danger is performed. Pessimism and depression,however, may be associated with "excessive" worry and rumination. In such situations the result can be paralysis and potential self-destruction.
Likewise, optimism when carried to the extreme can lead an individual to ignore potential danger resulting in foolish and potentially self-destructive behavior as well.
Once again, the approach should be to strive for the middle path. Explore the extremes but attempt to return to a balanced approach.
Easier said than done but it does recognize the value of "negative" traits within all of us.