Why Do Teenage Girls Get Anorexia?

They start as an insecurity and transform. It can be the result of bullying, rejection, or any other type of abuse (physical or mental). Since the world has become so dependent on outward appearance, teenage girls feel that they must be skinny and look like the models. This can turn in to anorexia, consciously, or subconsciously.

Unfortunately society puts it into their heads that they are fat. That they should be a model size 0 or a size 2. Nobody should be that small unless they are naturally made that way.No one should be obese because it's detrimental towards their heart and other organs and so is anorexia. We need fat to be stored in the body. If there is no fat your body feeds on itself.
Women need to wake up, stop trying to be a model or imitate them, eat healthy, neither too little or too much. In actuality we are suppose to eat 5 small meals per day. Eating one meal can mess up your metabolism and make you GAIN weight.

Well there are lots of obvious reasons why teenage girls get anorexia, while Speculation and scientific research all try to understand and explain the causes of eating disorders – it is unknown what exactly triggers or causes something like anorexia for example,
but possible explanations that are offered include: 1) feeling stressed out or upset about something in your life, or 2) feeling like you need to be "in control."  3) Society also puts a lot of pressure on people to be thin.
Anorexics usually think they're fat, even though they're very thin. People with anorexia may get so thin that they look like they're sick. This physical manifestation of harmful intent to self, threatening all-round health and wellness and total well-being is disturbing and also prevalent in the teen population. Individuals suffering from anorexia, go to extreme lengths to keep themselves from gaining weight.
The alarming effects are, 1) 8,000,000 or more people in the United States have an eating disorder, 90% are women. 2) Victims may be rich or poor.  
3) Eating disorders usually start in the teens but may begin as early as age 8.
Source: Teen Anorexia

If its not a cause of any other disease.. Then its what we call mental disorder.. TEENs who are anorexic or also called anorexia nervosa feel and think that they are TOO FAT even if they are all skin and bones.. The problem here now is self image disturbance.. They don't want to eat because they don't want to go more fat as they think they are.. And yes they can be classified crazy..

LOT'S of reasons. Mostly because of kids at school making fun of their weight and stupid modeling.
And girls get way over their heads and tend to believe that not eating at all will give them that slim healthy look. Which is totally wrong.
All girls have different frames and weight and everyone is beautiful. And most of the time girls don't get as slim as they want. So what? That's not stopping them from living their lives unless they let it. They best way to look and feel great is the exercise every day and eat healthy.
And trust me, I KNOW everything about eating disorders, I used to have one. Last month actually. Passing out in Gym after not eating for an entire day knocked some sense into me.

Look around. Everybody and their mother think the most beautful person is a skinny one. Bullies and or family members can allso be to blam. If someone is all ways talking to them about how they hate fat people or telling them that they are fat and they need to be skinnier and this person is someone they hold in high regards or someone they see everyday or every other day due to school, work, ect ect then this becomes imbeded in their mind and this is where this problem comes from

Most likely because most teenage girls are obsessed with having a slim body and sometimes their obsession goes to their head that they can't possible be slim enough so they stop eating and lose so much weight that their health is in danger.