Maybe your grandparents lived to 100 and you take good care of yourself. You're healthy. But if you want those years to be full of smiles, you need to invest in friendship. 70% of your happiness comes from relationships.

Via The 100 Simple Secrets of Happy People :

Contrary to the belief that happiness is hard to explain, or that it depends on having great wealth, researchers have identified the core factors in a happy life. The primary components are number of friends, closeness of friends, closeness of family, and relationships with co-workers and neighbors. Together these features explain about 70 percent of personal happiness. - Murray and Peacock 1996