Michael Pollan, author of "The Omnivore's Dilemma," points out the possibility that the reason Americans have such difficulty maintaining their weight is there are no cultural traditions or taboos surrounding food/mealtime. The French, for instance, apparently will not go for seconds and will eat only small serving sizes, although they will certainly eat (and drink) whatever they want. They also have a culture that places value on eating together. Americans have become a people that have no issues with eating meals in the car and/or eating alone.
I agree with him, but would add that Americans are also an overworked people who get two weeks of vacation per year, compared to month-long holidays the French (and many Europeans) are accustomed to taking, or so I hear. This sort of thing does not make for happy, healthy eaters in America.