Why can't we eat just one?

I am talking about the fat foods in life. Can you eat just one doughnut? Can you eat just one potatoe chip? Some people, with a strong determination, can eat just one of just about anything and let it go at that. Unfortunately, I am not one of those people. My inner cravings will not let me stop at “just one” of anything that has to do with food. Am I overweight? Am I piggy? I am fortunate that my metabolism burns away excess fat and allows me to keep my ideal weight, at age 67. Am I piggy? Probably. If it were not for my inner weight control, I would weigh much, much more. Question: how about it? Can you eat just one doughnut and walk away?