Tastes Better Than Medicine!
It's been psychologically and scientifically proven that people with healthy mental and emotional states have healthier bodies and lives, and they get over illnesses faster. Laughing keeps you mentally and emotionally healthy. Therefore, by transitive property, laughing keeps your body healthy! :)
I had depression for many years and I was always sick. Since I got treatment I am only really sick about twice a year.
Besides... laughing is just fun. :)