Its also have something to do with genes. If your parents are both slim, then there's a bigger chance that their children will have a slim body as well or thin. One thing also is because you have a fast metabolism wherein even though you eat more rice or carbs, you probably cannot gain enough weight and fats. Well, I think the better ways for you to do are to drink vitamins, drink whole fat milk, eat foods that are rich in protein, more carbs and less snacks.

I think you need to drink vitamins or food supplements in order for you to crave for more food and for you to eat more than before. Also, milk can help you gain weight and other dairy products.