Answer: Insulin plays a major role in keeping us alive, but in short, this hormone is not responsible for weight loss or continuous body fat gains. Only we are in control of our weight. Whether one increases or decreases the size of their fat stores from day to day depends upon the relationship of calories eaten to the amount of calories used through metabolism and daily activity. If, at the end of the day, you are in a caloric deficit, more fat is burned than is stored, and fat stores will decrease. However, if calories eaten exceed calories used at the end of the day, more fat is stored than burned, and overall stored fat increases.
Humans, as periodic eaters, need insulin to survive. Insulin helps the body store energy to fuel the body’s continuous needs and activities. Insulin is secreted after eating in order to move energy (e.g. glucose, amino acids) into the liver, muscle and adipose tissue (fat)—the body’s primary fuel source—for storage. Within about an hour after a meal, insulin levels diminish, leading to an increase in the hormone glucagon. Glucagon signals the body to begin releasing stored energy (glycogen from the liver and muscle, and fatty acids from adipose tissue) into the blood stream to fuel the body’s continuous energy needs, essentially reversing the actions of insulin. This cycle is repeated with every meal.
Essentially, insulin is a guy doing his job, which is storing things. However, it is the person, through eating, who gives insulin the "things" to store. In other words, insulin does not cause a person to become fat. The excess food one consumes leads to the average adult’s growing waistline, and of course that is 100% under the control of the person eating the calories.