What makes it difficult is that it's not any one of those things--it's ALL of them.
We eat crap, we eat lots of it, we eat it as fast as possible, we like getting more of it for our money, and we children of Depression-era parents were taught to clean our plates. Big business has figured all of this out, and bends over backwards to sell us more crappy stuff to eat, as cheaply as possible, and as quickly as they can get us in and out the door. The cycle escalates every time the super-size becomes everyday in consumer perception.
We spend all of our time at couches and desks and in cars, so activity is decreased even as our consumption increases. Lawsuits for everything are the rule of the day, and personal responsibility is out of style. The Food Network shows us all the yummy stuff we should be learning to cook, while diets are hyped from every direction, promising everything from quick and pain-free miracles to complete lifestyle overhauls.
And that's all without the added complication of eating disorders, body image, social perceptions of attractiveness and norms, and all that baggage.
Really, it would be amazing if we WEREN'T becoming the Land of the Free and Obese. You want fries with that?
That's a good point too, about what we're teaching our kids about how to eat. It's not just the fast food, either. Look at any kid's menu at nearly any restaurant, and they're the same: hot dogs, fried chicken fingers, grilled cheese, and maybe spaghetti, quesadillas, and hamburgers. After all, we can't expect our toddlers to eat real adult food--they'd make a mess! they won't want it! it'd cause trouble! So we give them high-cholesterol, high-fat fried crap--sometimes at home, too. That's really beginning to annoy me every time I'm out to eat with my two-year-old nephew: there are no healthy choices compatible with his skill levels with utensils, no matter where we go.
Oops, got into rant mode. sorry.

Okay, I'm done.