Complexity – everywhere complexity
When it comes to such complex systems as the human body, I doubt there could be a single root cause for a problem like obesity. I know Gary Taubes (among others) tends to see the argument that obesity is multi-factorial as an excuse to do bad science. But even though much of what the average layperson knows about obesity and how to deal with it is inadequate and often based on bad science, that does not mean the problem of obesity can be reduced to one root cause. Reducing complex problems to the most simple explanation works better in math and physics; it does not work so well with human biology. Perhaps the best chance for success in treating obesity is to address the problem from many angles at once – diet, exercise, stress management, improved sleep, treatment of deficiencies, treatment of emotional problems, etc.