A disease can be self-inflicted, have symptoms, and can result in death. Things like cancer can be caused by smoking, exposure to radiation, and even exposure to building materials. That doesn't mean it's not a disease.
According to the Mayo Clinic, obesity is classified as a disease. This is as it is not just a cosmetic issue, it puts a person at risk for other complications such as heart disease, diabetes, high blood pressure, and certain cancers. Contributing factors can be socioeconomic status, as well as eating habits. In addition, it is not necessarily just a lifestyle issue that causes the disease, genetics can influence it.
Cancer itself is not something that would cause an issue, it is how that cancerous tissue affects other tissues in a person's body. The same thing goes for obesity, the symptoms of the disease affect how well other parts of a person's body function.
Many studies have been done to look at what effect exercise and eating healthy have on obesity. It has been found that in those that face prolonged obesity, these actions can have little effect.
This implies that though exercise can help some people to live a healthier lifestyle, many people struggling have other factors contributing to their obesity. Though the exact cause of their obesity is unknown, obesity itself can be considered a disease.