What happens in the brain when we do too much of a good thing...

How about a definition of addiction from Joseph Frascella, director of the division of clinical neuroscience at the National Institute on Drug Abuse.  "Addictions, are repetitive behaviors in the face of negative consequences, the desire to continue something you know is bad for you."  Investigators are researching, through MRI and PET scans, what happens to the brain with addictions.  The brain gets hijacked in certain regions and this in turn affects the memory making processes and exploits emotions. 
In addicts, the natural damping circuit called GABA (a neuro-inhibitor) appears to be faulty.  Without the inhibiting effect of GABA, the brain doesn't know that it has been satiated.
There is also evidence that the prefrontal cortex in some people, that have been using methamphetamine, can be inhibited.  There is less activity in the decision making area of the brain, so there is less ability to use cognitive skills and adjust to new rules.  These people had a higher chance of relapse.  These tests were conducted in a VA hospital with a 4 week intensive program.  Dr. Martin Paulus, a professor at University of California, says "You have a better chance to do well after many types of cancer than you have of recovering from methamphetamine dependence."
These people are struggling.  Send them my way.  People can do better with LENS neurofeedback.