Dr. Phil says obesity is a disease. Other than legitimate medical conditions, isn't getting fat a choice?

There are many reasons, just like for alcohol consumption.  Some people can eat twice or three times as much and not gain a pound.  Some people have slow metabolisms.  

Most people do not understand what a disease even is.

If your body is not operating harmoniously, some kind of disorder, usually some kind of disease is happening.  It can be of the body, or mind or even spirit.  All three must function harmoniously or something negative transpires.