Happiness -- you know it when you see it, but it's hard to define. We also know that we don't always have control over our happiness. Research suggests that genetics may play a big role in our normal level of subjective well-being, so some of us may start out at a disadvantage. On top of that, between unexpected tragedies and daily habitual stress, environmental factors can bring down mood and dry up our thirst for living. Being able to manage the emotional ups and downs is important for both body and mind, said Laura Kubzansky, professor of social and behavioral sciences at Harvard School of Public Health. Many scientific studies ... have found a connection between psychological and physical well-being. A 2012 review of more than 200 studies found a connection between positive psychological attributes, such as happiness, optimism and life satisfaction, and a lowered risk of cardiovascular disease. Lower blood pressure, normal body weight and healthier blood fat profiles were also associated with a better sense of well-being in this study. Some researchers speculate that positive mental states ... have a direct effect on the body, perhaps by reducing damaging physical processes.