People with eating disorders tend to be perfectionists and may thus display unrealistic expectations of both themselves and others. In spite of their many achievements, they feel inadequate and may focus on their flaws and imperfections. In addition, they may see the world as black and white with no shades of grey. Everything is either good or bad, a success or a failure, fat or thin. If fat is bad and thin is good, then thinner is better, and thinnest is best.
People with eating disorders may also be struggling with establishing a sense of identity. They may thus try to define themselves by manufacturing a socially approved and admired exterior. Even if they may want to appear strong, inside they may still feel weak and powerless. An eating disorder may also be an expression of strong emotions like anger which the person does not dare express directly and not know how to express in healthy ways. They may then turn it against themselves by starving or bingeing and purging.
For someone with Anorexia, for example, achieving and maintaining stick thinness is a badge of honor worn with pride by those who can do it. Even if they have earned excellent grades or career honors, being thin may be the only achievement they define as significant. They are not happy and, in fact, may be miserable, but they cling to this achievement as if it validates their existence.