I've been normal weight my whole 33 year old life and I could easily wolf down many thousands of calories per day if I didn't make a meal plan for myself. The human body is designed for conditions that existed thousands of years ago, that's why. It does take initial planning and effort, like anything important. What's different about fat people is whereas we can ignore our caveman taste buds and retrain our palate, they still act like a caveman (and smell like one too).