The old hindbrain keeps us breathing, balanced and alert, just as it did our ancestors a half-billion years ago. The midbrain, built atop the hindbrain, does things like manage vision and hearing. The forebrain, the last addition, deals with language and decision-making - but not without the input of the other two regions. So the even more recently evolved "deliberative system," which works hard to defer the gratification of cheesecake in favor of your long-term weight-loss goal, is easily overwhelmed by the ancestral "reflexive system," which lives for the moment just in case those 800 calories are the last ones the organism will see for a while.