Although the belief in a better future is often an illusion, optimism has clear benefits in the present. Hope keeps our minds at ease, lowers stress, and improves physical health. This is probably the most surprising benefit of optimism. All else being equal, optimists are healthier and live longer. It is not just that healthy people are more optimistic, but optimism can enhance health. Expecting our future to be good reduces stress and anxiety, which is good for our health. Researchers studying heart attack patients have found that optimists were more likely than nonoptimistic patients to take vitamins, eat low-fat diets, and exercise, thereby reducing their overall coronary risk. A study of cancer patients revealed that pessimistic patients under the age of 60 were more likely to die within eight months than nonpessimistic patients of the same initial health, status, and age.
Tali Sharot in The Science Of Optimism, cited by Maria Popova in Brain Pickings
I also believe that optimists have better sex. Just saying.