system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Why is Anexoria a disorder but Obesity isn't?
gpt
Because Anorexia is not being thin. It is a disorder where someone think they are never thin enough. 

Someone who is obese because they suffer from clinical depression, or a compulsion to eat, certainly does have a disorder.  Just like someone who is very thin, but does not have a controlling belief that they can never be thin enough and a fear of being fat is not anorexic. 

The difference is that there are a lot more ways that someone can end up fat to an unhealthy degree without a specific mental disorder, but very few ways that someone can end up as unhealthily thin as someone with anorexia without such a disorder.