I read a book a while back called "Evolve Your Brain" by Dr. Joe Dispenza. In it, he made the case, backed up by scientific research, that it really is a form of withdrawal we go through when we change our behavior.
In short, the brain stimulates various glands in our body, which secrete various hormones (chemicals), that surge through our bloodstream and affect our bodies. The cells become accustomed to receiving these hormones and alter themselves to better receive them. When we change our behavior, the brain operates differently and triggers different hormones/chemicals to be released. The cells are used to receiving their regular "dosage" of certain things, and are not at all prepared for something different. So, until our bodies catch up and adapt to the new mix of chemicals, we feel the urge to "go back" to the way things were rather than face the discomfort of changing.
Really interesting reading.
L