Why can't I be skinny?

The simplest guideline is this - calories in has to be less than calories out. If you are eating more calories than your body can use it makes fat... simple as that. Absolutely no mystery to it. Eat the foods that have lower calories and you can eat more food. Make a lifestyle that doesn't revolve around food. Only do exercise that you like. I only know of one person who actually likes gym and makes regular happy visits to it. Walking half an hour every day is heaps better than killing yourself at a gym three times a week. Best of luck...