its because its a very addictive substance which you WILL grow to enjoy more and more, and well once you start doing that you'll get problems, its like cigs, who liked there very first cigerette? bet not many, i didnt, but now i can barely stay a day without one, i would rob off some one to get a pack of fags if i went cold turkey for a few days!!!
i dont know how addictive compared to cigs it is, but im sure its close, and imagin ppl who smoke, instead of smoking nicotiene they were smoking heroin which is much more expensive, that would start to ruin your life, would it not?