Dr. Neil Barnard, who runs the Physicians Committee for Responsible Medicine, has started a vendetta against cheese. Cheese is so addictive, he said, because it contains the protein casein, which creates casomorphins when the dairy breaks down in your body. "Casomorphins attach to the brain's opiate receptors to cause a calming effect in much the same way heroin and morphine do," he told the Vegetarian Times. "In fact, since cheese is processed to express out all the liquid, it's an incredibly concentrated source of casomorphins, you might call it dairy crack." Further studies still need to be done to show exactly how casomorphins affect the brain, so it's not set in stone yet. Plus, our bodies actually love cheese, since it gives us nutrients, which sets it apart from, well, actual crack.
http://www.delish.com/food-news/a44472/cheese-addictive-study/