We have children facing social rejection, failing/dropping out of school, becoming dependent on drugs, suffering from depression/anxiety and other mental illnesses, committing suicide, etc.
In a perfect world we don't have to change our lives to fit into society at all! Unfortunately, the world isn't perfect. We have an obesity epidemic because people sit in an office working all day. One treatment I'm sure you'll approve of is going to the gym. Unfortunately gyms are pretty unnatural.
Why should obesity be considered a disease in any objective sense? People are suffering because modern life is sedentary. It doesn't mean there's something wrong with them.
—We change our behaviors to make up for the personal problems that come with society. We do unnatural things like go to gyms and take medication to help our bodies (which were made for a world where we live in caves and hunt wild animals) function properly in this society, because we agree its advantages vastly outweigh its disadvantages.