That's an easy one, but I warn, I can talk a long time about it.
I want you to imagine a patient with severe obsessive-compulsive disorder, who has the constant thought, “There are germs on my hands,” and the only thing that makes it better is if that patient goes and washes their hands, and then the anxiety decreases. But that patient sits down and the thoughts appear again, “There are still hidden germs on your hands. You didn't get them all. They're going to get inside you. They're going to make you sick. They're going to make you die.” You can't stand the agitation, and the only thing that makes it better is washing your hands. Individual treatment would be like trying to get the person to choose not to wash their hands. Even if they're motivated to do that, they probably can't tolerate the severe urges.
Anorexia nervosa in youth is the exact same kind of illness where they can't tolerate the severe anxiety and agitation that goes with the thoughts that have taken over their minds that constantly say, “You're eating too much. You're gaining too much weight.” They feel compelled to restrict or purge, or whatever, to get rid of that. You can't just talk them into not doing it. First of all, they're not motivated because they're afraid of gaining weight. Secondly, even if they were, they can't tolerate it. So we put their renourishment into the hands of parents whose job it is to prevent the opportunity to have symptoms.