"Fast Metabolism" Is (Mostly) a Myth
Your metabolism is individual to you, based on a combination of your genetics, age, gender, muscle mass and certain environmental factors. While your best friend who seems to be able to eat anything and still stay slim seems to have a faster metabolism, it's not likely her metabolic rate creating such a difference. She's probably just found, either through work or good luck, the perfect ratio of lifestyle factors. She's working with a balance of how many calories she eats, how much she exercises, how much muscle mass she has and how much she sleeps. If she has that piece of pie after dinner every night without gaining a pound, she's probably also already slender, very physically active and getting lots of uninterrupted sleep every night.
When you skip meals or severely reduce the number of calories you eat, your body compensates by slowing down your metabolism, allowing it to save calories for energy your body will need to handle its basic functions. When you eat too many calories without also increasing your physical activity, those unused stores mean weight gain.