In the December issue of Educational Leadership you'll find a great article on physiologically why we over-eat. It is called, "Finding Our Way Back to Healthy Eating: A Conversation with David A. Kessler."
Kessler points out that "forty or 50 years ago, the U.S. food industry shifted from food that was locally grown to a highly interdependent food distribution network that had many advantages in terms of cost and food safety. But now that food is so highly processed, we're able to dial in the exact amount of fat, sugar, and salt that will make us come back for more." He talks about the addictive effect of sugar, fat and salt. And combinations of fat, sugar, and salt drive consumption—they stimulate you to come back for more.
Check out complete article or listen to the podcast from the link above.