I'm sure serial killers say the same thing //content.invisioncic.com/y282845/emoticons/rolleyes.gif.c1fef805e9d1464d377451cd5bc18bfb.gif As well as rapists, theives, child molesters, etc etc.
Sorry, but that's a pretty weak argument. "Why should you decide what's best for me?" Because that's part of the governments job; deciding what they do and don't want their public citizens doing, or how they do or don't want them behaving, and then passing laws accordingly. If you don't like it, go find a country that allows you the "liberties" you want.
But if you want to use that argument, "Why impose your wishes on my body," why stop at weed? Lets go ahead and legalize crack, heroin, Exctasy....all of them. I mean, that's us imposing our "wishes" upon their body, right?
And if serial killers want to kill, lets allow them to. I mean, who are we to decide what they can and can't do? Lets allow child molesters to run free because we shouldn't decide what is wrong or right for them to do.