Carbohydrate is the nutrient that changes its composition during cooking that will give you a natural sweetness to food that you eat. Well, carbohydrates are composed of starchy and dairy materials that naturally secrete, or produce, sucrose (a compound that is the chief/main component of cane or beet sugar) in certain amounts. When it is cooked, these sucrose are secreted and the food gets its naturally sweet taste, which is pleasant to the sense to not only the taste but to the smell as well. This is the reason why when you smell food rich, or plentiful, in carbohydrates, your brain signals you to eat it, which gets you excited.
