T%hey can't imagine that a person exists who doesn't spend every waking minute (and many sleeping minutes) thinking about food, so they think in order to be thin you have to live in a constant state of starvation and denial. Then when they see evidence of that not being the case--when a thin person eats a piece of cake or a burger and fries or a candy bar in front of them--they decide it must be genetics.  They are simply incapable of understanding portion control and/or that you can stay thin and still eat a perfectly satisfying amount of food, with dessert and snacks if you wish, as long as your "perfectly satisfying amount of food" isn't pounds and pounds of it per day. It never occurs to them that you do not have to make a choice between eating foods you enjoy and being an enormous gluttonous pig. You can eat foods you enjoy and still be thin.