Previously we mentioned that the pleasure center is a portion of the brain over which we have no conscious control, and that it can be stimulated by a variety of chemicals — some of them produced inside our bodies and some that we introduce from outside.  We said that the pleasure center rewards us for activities that it interprets as contributing in some way to our survival, whether they be social interactions, exercising, or more prosaic things such as eating.  We also stated that these pleasurable feelings, when pursued too far or for too long can create problems.  Now we need to examine how that happens….

http://sunrisedetox.com/blog/2011/08/24/addiction-alcoholism-compulsion-2/