Why does the weight come back so quickly?

To answer this we need to understand how much energy a body requires. For each pound you weigh, each day you need 12 calories to maintain your body weight. If you weigh 120 pounds you will need 120 x 12 calories, that is, 1440 calories per day to maintain that body weight. If you eat or drink more calories than your body requires, the excess energy is stored as fat. It takes 3,600 excess calories to make one pound of fat.

If you want to keep the weight off you must develop a consistent change in eating habits to ensure you do not consume more than your body requires. You cannot continue to eat the same quantities and/or combinations of foods that caused you to be overweight in the first place. This will require developing an understanding of the nutritional content of food and raising your body's metabolism through increased muscle mass and exercise.