It's just that I have had so many friends who have been totally level-headed most of their lives, and then become addicted to whatever (normally drugs) and morphed into total shit-heads. They know that what they are doing is going to kill them eventually, but the rush that they get from the addiction out-weighs the dangers of it.

I know drug addicts, alcoholics, fruit-machine addicts, exercise addicts - all of them are sane people, and they all know that their addiction is going to become detrimental to either their health or their finances, and yet they don't stop. You just can't stereotype an addict, you can't label them all as 'stupid', it's far more complex than that.