According to data from two large studies spanning thirty years, optimistic people live considerably longer than pessimists.
Why? Apparently optimistic people eat better, exercise, manage stress more effectively, and generally lead healthier lives. Well goody for them. From Science:
…The scientists were able to show that the most optimistic women (top 25%) lived an average of 14.9% longer than their more pessimistic peers. For the men the results were a bit less dramatic: The most optimistic of the bunch lived 10.9% longer than their peers, on average, the team reports today in the Proceedings of the National Academy of Sciences.