The brain knows that when a person eats, they're doing something right, and it releases feel-good chemicals in the reward system. These chemicals include the neurotransmitter dopamine, which the brain interprets as pleasure. The brain is hardwired to seek out behaviors that release dopamine in the reward system. The quicker the carbohydrates are digested (especially in the absence of fat and /or protein), the higher the rise and fall in blood sugar, and the greater the effects on energy and subsequent cravings for food (and 'lack of willpower').