Of course people don't "go" anorexic on a whim.
It's not exactly a lifestyle choice despite what the pro-ano websites will tell you. A lot of these sites are actually set up by wannabes, cultural trend victims and bandwagon jumpers.
Here are a few random facts and theories about anorexia:
Many anorexics are victims of child sexual abuse.
Anorexia is a psychiatric condition and a mental health issue, but not a mental illness. Anorexia can't be 'cured'. Like alcoholism, it's with you forever and it can only be managed.
For some, starving themselves is a metaphor of wanting to "disappear" from a male dominated society. For these people it's about denial. Denial of food equates with denial of sexual appetite, denial of one's right to exist (this is the feminist discourse)
For some, anorexia can be triggered by a consumer society that equates "skinny" with "beauty" and foists unrealistic aspirational images on young women in movies, magazines and the media (Marxist discourse)
For some, self-starvation is an attempt to stave off sexual maturity by maintaining a prepubescent body and rejecting "becoming" their mother (Freudian discourse)
For some, anorexia is about self discipline, denial and punishment. Starvation can also induce a sense of euphoria, much like meditation, or running a marathon (the aescetic's approach to eating disorders)
For some, it's a 'control' issue. Some anorexics substitute controlling what they eat for a sense of control over their lives. They reason, I mightn't be able to control other aspects of my life, but at least I can control what goes into my body.
As you can see, anorexia is no simple matter. It's complicated.