I think that it depends. (Do you like my lawyerly answer Meg?
For some obesity is a purely biological thing. They are the ones who literally eat clean and exercise and their weight is still high.
For some, obesity is a matter of culture. These are the ones who may be biologically pre-disposed to obesity, but that being in a culture with very cheap and accessible ever so tempting high fat food just leads them awry.
For some, obesity is a psychological issue. These are the folks who are emotional eaters and/or addictive eaters and that need their underlying psychological issues to be identified, dealt with and constantly managed in order that the weight may stay off.
For many, obesity is some type of combination of the aforementioned.
For myself, I consider my obesity to be a genetic predisposition but one that was aggravated to an astounding degree by my psychological issues. Once those issues were identified and managed I was able to eventually move on from using food as a source of self-medication, a crutch and a means of hiding myself from the big, bad, scary world. And as a result, the pounds came off.