Your Genetics
Your Diets
Your Workouts
Your Routine
Your Atmosphere etc.
Making muscle is a mind game as well physical game. If you are a fatty guy, then you'll have to work hard on being muscular man in Diet, Workout, Routine and Atmosphere ways because your Genetics are not like that.
If You're a thin guy then you'll have to work only on your Diet and Workout, because being metabolic is in your Genetic.