You're not giving sufficient emphasis to the distinction between the motivations which lead to the disorder, and the disordered thinking itself. Regardless of where something like anorexia nervosa comes from, the bit that makes it a mental disorder is that it produces irrational behavior. It's that distinction which most people fail to appreciate. Nearly all of us are exposed to insatiable peer pressure, unrealistic social norms, and mooks, but some people get it worse than others, or can't cope as well, or have a genetic predisposition, and develop a disorder. The disorder itself, as with any medical or psychological condition, has, as Sajainta hinted at, little or no connection to the impetuses which caused it.
