What is the reason behind the scenes that makes the body stores fat instead of burning it and get rid of them?

 I do not expect that fat storage in the body due to eating will decrease or increase! Why Because I am with my brother on one trip and he eats less food than me, and the family knows that. However, he is fatter than me, although I eat more of him !! What are the circumstances or situation Alaztarara to speak, surrounding the body were ill health, psychological, social or ..... or .... etc Tadjalh gives a command to store Aldoho in the body instead of burning and disposal.