Healthy food is about much more than obesity prevention. Yet, in its place, without obsession, obesity deserves a place on the list of nutrition concerns, not for cosmetic reasons, but because it is connected to diabetes, heart disease, and other chronic diseases.
The leading theory is that increasingly abundant and inexpensive and palatable food, combined with lower physical activity, has produced rising rates of overweight and obesity. If you just look at the changes in the food system, restaurant industry, sweeteners, sedentary lifestyles, and food advertising to children, it is entirely plausible that calorie balance explains the changes in obesity. In addition, there are a hundred theories about the one thing that obesity is "all about" -- maybe it's the insulin, or the HFCS, or the carbs, or the meat, or the fat, or the chemicals, or the lack of fiber, or the glycemic index, or the volumetrics. Proponents of each of these theories will admonish us to read "the science," by which they mean "just part of the science, please." My advice is to follow each of those theories as their evidence base develops, but don't throw your weight or authority to any of them yet.