Eating disorders aren't only socially-rooted

If eating was just something that we did for fun, then yeah, eating disorders would be a choice. They would be something that people do just to be popular, 'lets just stop eating' and 'I don't need to eat today' would be common themes of high school conversation. But here's the thing, eating disorders are not just a fad or anything that people do for fun; they're real and they're a problem.
There are two main forms of eating disorders; anorexia and bulimia. Microsoft Student 2006 defines anorexia as:

Mental illness in which a person has an intense fear of gaining weight and a distorted perception of their weight and body shape. People with this illness believe themselves to be fat even when their weight is so low that their health is in danger. A person with anorexia nervosa severely restricts food intake and usually becomes extremely thin.

Similarly, Microsoft Student 2006 defined bulimia as:

An eating disorder in which persistent over concern with body weight and shape leads to repeated episodes of binging (consuming large amounts of food in a short time) associated with induced vomiting, use of laxatives, fasting, and/or excessive exercise to control weight.

Anyway, I got on this topic because in AP Psychology the other day, we were talking about eating disorders. Although our book came to no clear conclusion, my teacher did, and this was what made me mad. He decided that the root of all eating disorders was the need to belong and the desire to be popular. This made me so mad, however, I didn't express my ideas because I rarely speak up in that class.
He didn't even make mention of problems that are obviously going on psychologically. (And by the way, the Microsoft Student definition of anorexia, which calls it a 'mental illness' is out of date because the term was recently dropped.) I know for a fact that, while there are some people that have strange social motivations, not everyone conforms to our preconceived stereotype. It is ridiculous to believe that there is a social root to every problem, because removing them from the social atmosphere and placing them in one that everyone was obese would not solve the problem.
And the National Eating Disorders Association agrees; on their website they posted:

The National Eating Disorders Association (NEDA) is a non-profit organization dedicated to supporting individuals and families affected by eating disorders. We campaign for prevention, improved access to quality treatment, and increased research funding to better understand and treat eating disorders. We work with partners and volunteers to develop programs and tools to help everyone who seeks assistance.In 1999, NEDA established a toll-free helpline and has assisted more than 50,000 people find appropriate treatment. Tallying more than 50 million web hits each year, NEDA is proud to serve as a clearinghouse of information on eating disorders.

Questions PSA from NEDA on Vimeo.
Most just have issues within themselves that they need to work out, and they don't need people preaching to them about how being popular isn't everything while they are trying to get better. Thus, we need to change our mindsets before we expect the problem of eating disorders to resolve themselves.