Why we lose more weight during first weeks of dieting?

This is not fat loss. It is carbohydrate and water loss

Our body stores reserve energy as Glycogen (Carbohydrate) in the liver and muscle. When our diet falls below the calories needed to sustain our weight, the body first uses up this Glycogen reserve. Glycogen binds 4 times water by weight. So, if we burned one pound of Glycogen, you will also lose about 4 pounds of water, resulting in a total loss of 5 pounds. But, this is not fat, it is just carbohydrates and water.