When one has anorexia as a teenager, they could become overweight when they are an adult and finally start eating. They usually have slow metabolism, then, because their bodies do not know how to process the food properly. Simply put, their bodies are not used to the food. Why do girls become anorexic in the first place? This is something that puzzles some, but I know why. Below, I am going to give you 7 reasons why girls become anorexic.