I mean, duh?<p>Eating makes you feel good. It releases all kinds of feel-good chemicals in your body, and it just feels good to be, if not full, satiated. It's an activity chock-full of positive reinforcements.<p>Exercise, in contrast, is filled with various forms of pain (short and long term) and lots of tedium. If you're lucky, you get an endorphin rush to help offset these, but not everybody (and not every exercise regimen) provides these. It's an activity with lots of built-in negative reinforcements.<p>And dieting? Dieting is constantly saying "No" to the positive feelings associated with eating. A piece of Christmas fudge? No. A side of fries with your hamburger? No. A full rack of sauced BBQ ribs? No. Creamy dressing for your salad? No. Seconds? A whole lot of No.<p>It takes a lot of willpower and a long term goal to maintain a certain body shape to consistently - over the course of your entire lifetime - exercise more and eat less. And any mistake, any injury or illness which pushes you into obesity, and you're having to go even further into the "this doesn't make me feel good" category.<p>Throw in a continuing shift towards jobs which don’t burn thousands of calories every day (e.g. forced exercise, overhead for increased caloric intake), and the availability of cheap meals with lots of calories, and boom. Obesity.