I believe it is a combination of everything you have all mentioned..the fast food, the huge portions, the addiction to sweets and carbs, and the sedentary lifestyle. Kids and adults are glued to tvs and videogames and computers now instead of playing sports, working outside, and other healthful pursuits. People that smoke are even more handicapped, as the smoking takes away their full capacity to take in pure air, draining them of the energy to do their work or sports. We have become a pleasure oriented society. IF it feels good or tastes good, do it. We are killing ourselves with kindness.
I'll get off my soapbox now. I'm just as guilty as the next person. I'm sitting at my computer instead of getting my housework done.