Surprisingly, cheerfulness is not the personality trait that predicted long life, but conscientiousness. There are three possible reasons suggested by the author. The first one is that conscientious people are less likely to do things that are detrimental to their health and well-being. Which means that they are less likely to smoke, drink to excess, abuse drugs or drive too fast. Second, conscientious and unconscientious people have a different level of certain chemicals in their brains (like serotonin). People with high level of conscientiousness are also less likely to suffer from more serious, chronic diseases.
It is not unlikely for us to change our personality. But we can only work slowly with it. One of the participants in the Terman project, James, goes from the bottom 25 % in the conscientiousness test to the top 25% over a decade.