Why?
EDs tend to have a lot in common. There are several reasons/causes behind them. These aren't in any particular order, but I'd thought I'd address this one first. (It matches the image at the top of the page.) I'm going to name them all. It helps me keep them straight.
Lexie comes from a very strict family background, with high expectations for her and her future. There are a great many rules in her household, and the she is extremely overprotected. Most decisions- how to dress, where to go to school, what to take in school, who to hang out with- are made by her parents. Their expectations sometimes seem impossible for her. She feels inadequate and out of control. By restricting calories or purging she is suddenly in control again. It grows addicting, because this is the only area of her life she feels she has any choice in. The reality of the situation is that it controls her. It is a dangerous behavior, the side-effects of which will be explained later.
Ellen was a victim of sexual assault. She's seen pictures of anorexics, and knows how unattractive they are to most men. By starving herself and being rid of body fat, she knows that her feminine features (breasts, hips, posterior, etc) will all pretty much disappear. Somehow, she believes that making herself less physically female she will be "protected" from another physical assault. Again the truth is that sexual crimes are not about fat, thin, pretty, ugly- they are about power.
Helen grew up playing with Barbie, and reading magazines with waifish models. Even though she knows that Barbie is not real, and that most of her models were airbrushed, she still believes that she should look like that. In fact, Barbie, if made actual size would be seven feet tall, and have about a 21" waist.
Tonya is a dancer. She wants to become the world's most famous dancer, and dance on every major stage on earth. Traditionally, the image of a ballerina has thin legs, almost all muscle, thin arms, and a flat stomach. Sometimes you can even see the light shadows of their ribs through their costumes. Tonya is towards the top end of her healthy weight range, but thinks she must be at the bottom in order to get the big dance solos she wants so badly.
There is actually a genetic predisposition for EDs. Having anorexia in your family history puts you at 12 times the risk for developing it.
ED personality types:
People who suffer from EDs tend to be perfectionists, with the all-or-nothing attitude. ("I need to do this perfectly, or there's no point in wasting my time.") Sometimes EDs are secondary disorders, being derived from other things such as obsessive-compulsive disorder.
The start of an eating disorder-
Anything can trigger an eating disorder to begin. It could be something as (seemingly) trivial as being teased on the playground, or something as serious as a sexual assault. The start of an ED is usually triggered by a period of change such as going to a new school, getting a new job, or even puberty. They can also be triggered by shocks, such as sexual assault, or even by grief. EDs are like pathways which avoid the real problem, and put off facing said problem. Usually what links these things together, is that the individual is faced with something they are already unsure about, and they choose not to face said problem. Perhaps their significant other broke off the relationship, causing the ED'ed individual to question themselves further. Regardless, EDs are based around insecurity and low self-esteem, and are emotionally, physically, and socially damaging.
Some information taken from http://www.anred.com/causes.html