[cableshaft]

At least in part it's cultural. We work much longer hours with much less vacation days than pretty much the rest of the first world (http://abcnews.go.com/US/story?id=93364), often sitting in front of a computer all day, then go home too mentally exhausted to make good decisions about eating or exercise, driving in cars to go everywhere because everything is more spread out and very few cities are set up to make walking or biking feasible, so some people get very little exercise at all during the day, all while being served (and thus eating) larger portions of food at restaurants than most other countries.

I'm not saying we shouldn't exhibit greater willpower, but we're animals, and animals tend to be lazy when given the opportunity. If US cities (especially small cities and suburbs) were designed more to facilitate walking I think we'd be healthier overall.

I know in college I was in much better shape and I had to walk at least two miles a day to get to and from all of my classes (we had a large campus and a giant quad separating the buildings), and I often worked out at the gym since it was literally a two minute walk from my dorm.