Why do I gain weight so fast and easily ?

Usually after eating people have gained weight on the scales regardless of general fitness, because it's simply the weight of the food adding to your body weight. Try to limit things like pizza. Try to find more limited things that are similar to what you eat. Try pita pizzas with tomato sauce, fresh veggies and cooked chicken. Salads with cooked chicken or fish is best for lunches. Really butter/cheese and carbs like white bread and pizza/pasta are the worst. Don't eat anything white. The more colour the better. Don't eat after 8 (or if you need a quick bite on a random occasion- no carbs). Drink lots (2L) of water a day to clear system. Good luck.