Right down in the very roots of thing, an eating disorder is always about control.
@OwlBasket A lot of people feel it is for them but I don't think it particularly was for me (at least when it started.) I suppose the more extreme symptoms might be about trying to control my calorie intake/body size.
Anything could be called control I guess. Overeating etc is lack of control really IMHO, at least for me and a lot of others. I'm not trying to make myself less conventionally attractive by doing it. It could be partly because I have some ADHD traits so I don't have much self-discipline- I'm the same with sleep, ooh, I can stay up late- the delicious naughtiness and agency.
But of course my parents aren't in charge now, it's only myself I'm rebelling against now really, unless it's society.
I decided to starve myself to death at 13, as I didn't have any friends. Also a couple of family and family friends gloatingly delighted in commenting about my weight- I'd always been very skinny but filled out a bit as girls do. I was what was my adult height for quite a while, 5ft 3 and a whole 8 stone 3. 
Also my parents were tall and I didn't want to be tall, I wanted to be petite as I thought my mum's build was quite masculine (not fat but tall and robust-looking.)
I think part of it was my mum didn't give me enough sanitary wear and I was too shy about it to ask, so I wanted to reverse my periods (never succeeded even when I was eventually six and a half stone.)
Also with the bullying, being skinny was the only thing I did 'right' and wasn't bullied for, so I wanted to keep that thing. Same goes as an adult, it makes me feel less unattractive lol. I'm not quite overweight at the mo but would like to lose half a stone.
So those are some of the reasons I have some eating disorder symptoms. TL;DR. 
Anyone else?