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It isn't. Obesity is a medical condition in which excess body fat has accumulated to the extent that it may have a negative effect on health, leading to reduced life expectancy and increased health problems. So basically, obesity is a leading cause to diseases, not a disease itself.
However, it's important to note that many diseases can cause obesity. It would be more accurate to call it a symptom than a disease, really.
A disease is, as already stated, a condition. A condition can be a symptom, obviously.
It is a condition that (seriously) impairs normal functioning, thus meeting the definition of "disease."
The fact that something meets the current definition of a disease, doesn't mean that it's the most effective way of looking at the problem.
The disease-model may not be the best approach for many problems.
By considering it a disease it can have a treatment. For many people that treatment is diet and exercise, for some a gastric band or other.
however, some people may be obese due to a psychological problem, an unhealthy obsession with consuming food, which might be fixable by therapy.
Psychological therapy is often delivered as part of a treatment within a disease model.
Diagnosing specific problems can allow for targeted therapies, which can be more effective.