They enjoy the high and it's cheap. If they're not careful, they're addicted before they know it. I think you would be hard pressed to find anyone who says they wanted to have any kind of chemical dependency.
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It's not a matter of being careful or you might get addicted, it's a matter of not using it at all or you will get addicted. I developed a physical dependency for it over the course of a weekend. Thankfully I got out fairly quickly, but not before dropping to 120 lbs (I'm was about 5'11" at the time and 15 years old) -- hard to imagine it's been almost a decade already and I still get cravings.