When there is a lot of excess insulin and blood sugar in our blood stream, it signals our body to put that excess sugar in storage. We can store some sugar in our liver and muscles, however, when these are full our body start to store the extra sugar as fat. This of course starts to cause weight gain.
Furthermore, When the insulin levels rises, it puts the brakes on burning fat for fuel and encourages storage of incoming food, mostly as fat. That's why as long as the diet is high in carbohydrates, the body never has a chance to burn its own fat, making weight loss difficult. However, limiting carb consumption stimulates increased fat burning and decreased fat storage.