This is an important question because the answer you give can determine your success. Most people state they want to be healthy to avoid future consequences, such as a disease or premature death, but I find, from my experience as a physician, disease risk assessments are unlikely to motivate a person to be healthy. Calculating our risk for future sickness changes health from being a physical matter, one with inherent personal value, to being an intellectual matter, one aligned to abstract concepts. I find people are more likely to think their way out of health, than think their way into health.
Abstract health concerns are not very useful because our body resides in the physical world and subjected to a different reality. For instance, the Twinkie-induced comfort (the artificially derived feeling as a mixture of highly processed sugar and fat hit the brain) is very real in a physical sense, therefore not likely resisted for an abstract proposition such as to avoid diseases in the future. Moreover, we usually eat manufactured foods with impunity having calculated our odds of suffering based on the proclamation of wholesome goodness by a misleading "food" label. Eventually, we may feel ill and seek medical attention, but improving one's medical lab markers (another abstract proposition) provides insufficient motivation to rise from the comfort of one's favorite TV show and perform the labor of physical exercise. Intellectualization of health will not override comfort and convenience.
Health by physical means, by nature, automatically improves our sense of reality (our senses actually improve) and brings out the best of us, now and in the future. People who engage in healthy activities usually report a better physique, more energy, confidence, optimism, happiness, improved self-esteem and less stress in the present. BEING healthy generally utilizes skills and activities that immediately enhance the quality of our lives in a deeply personal sense, a biological sense. Consequently, future health is built upon present health.
We should THINK LESS and BE REAL. A diet of natural, whole foods combined with vigorous activity sustains the most sincere form of life, our most natural form of self-expression, one replete with intrinsic value and self-love. People tend to care for the things they value, therefore it is important to develop a body that feels valuable, a feeling that can only be created by the person. This is why we are healthy.