Anorexia first begins as regular dieting and food restriction,  and over time it can lead to extreme dieting and food limiting due to fear of getting fat. Another cause can be coming from a family with history of certain health problems.  These include weight problems, physical illness,  and mental health problems.  Mental health problems may include depression,  and substance abuse.  Things such as family influences,  Genetics, Brain chemical imbalances,  and development issues. 

  https://www.hopkinsmedicine.org/health/conditions-and-diseases/eating-disorders/anorexia-nervosa#:~:text=Experts%20don't%20know%20what,to%20fear%20of%20getting%20fat.