I would lay the blame largely on an odd intersection between our evolutionary heritage with modern capitalism's contributions toward the manufacturing and distribution of food. We are set up to naturally enjoy certain things which prior to industrialization weren't as widespread. Think on things such as sweets, 100 years ago they were manufactured largely by bakers/pastry chefs at prices which meant the average person wouldn't be buying them very often at all, cakes were made to be eaten over the course of days or with big celebrations. Now we are in a post scarcity age when it comes to food (of course modern distribution fails to dole this out to vast swathes of our human brethren), which in the case of our nation is a prime example of the ubiquity of cheap food and its effects on the waistline (not to mention the ego driven zeitgeist of always wanting more being praised by our social value system).
