I eat everything but why don't i became fat?

Family practice : It is because of your metabolism. Probably hereditary if you are trying to gain weight then see a nutritionist.

You're lucky: Some people are lucky, and have a higher metabolism burning calories, and do not gain weight. That does not mean you should eat whatever you want! it is still recommended to eat healthy, avoid fats and cholesterol rich foods, as heart disease and stroke risk can still be evident in thin people. Best wishes.

Difference : Different people utilize food differently. Some times young people can eat everything and stay thin and then after 35 eat the same food and gain weight. It is wise to eat healthy unprocessed foods with fresh vegetables and fruits and high quality meats, oils and starches regardless of your weight.