Obesity can often be associated with low self esteem, depression, body image dissatisfaction. As an adolescent, this is further exacerbated by being teased in school, not being able to perform as well in sports, and sometimes social isolation as a consequence of these.
Diet and exercise are the way to lose weight. Reducing calorie intake by decreasing dietary intake by a significant amount is the best way to lose weight. Exercise to burn off calorie, on the other hand, struggles to keep up to anywhere near impact of diet.
However, there are other more important benefits associated with exercise.