Pot is psychologically addictive in the way that alcohol is addictive - people feel they need it either in certain circumstances or to feel certain ways (relaxed, etc.). Recent studies show that pot is also physically addictive. Using a regular pot user who has just smoked up as a subject, researchers have shown that suddenly blocking the actions of THC (active ingredient in MJ) leads to withdrawal symptoms like those (though not severe as those) experienced during heroin withdrawal. And withdrawal possibility = physically addictive. Of course, most people never experience these symptoms because the pot stays in the system for so long. Of course, none of this is to say not to do the stuff, only that you should know what you're getting in to.