Frieder R. Lang, lead author of the study from the University of Erlangen-Nuremberg in Germany, said: “Our findings revealed that being overly optimistic in predicting a better future was associated with a greater risk of disability and death within the following decade." The corollary being - longevity increases if one is skeptical and a tad pessimists.
On the other hand, possibly the so-called "pessimist" were being realistic about the future. By expecting little from others there were not frequently disappointed. whereas, people who had high expectations often had setbacks when the reality was different.
It's probably worth reading the study for an in depth assessment of the results from the research study of Dr. Lang.