That's the the reason , The reason is the effects it has on other people , the community.
Don't be so single minded , Yes it's your body , what you do to your body can effect the people around.
So the well-being of society as a whole is more important than the freedom of the individual? perhaps in other parts of the world this is true, but I would argue that western civilization is extremely individualistic and libertarian. To prohibit such an act (the consumption of a substance) seemingly contradicts our most basic principles.
It's easy to say that "drugs are bad" - I generally agree. I've seen what drug addiction can do to a person, to a family, and to a society. I've lost friends to drug overdoses, and I've been personally effected by substance abuse in many ways over the course of my life, but to deny someone such a basic right requires much, much deeper contemplation than a superficial societal sense of right and wrong.
It seems that a lot of the responses in this thread are from young adults who are beginning to "see the light", as it were, regarding drug prohibition. The government's (and society's) approach to prohibition has been despicable at best. We're told from a young age that
all drugs are malignant tools of destruction and decay - a falsehood that inflicts irreparable damage to the psyche of some children.
When you're told as a young child that marijuana is a deadly, nefarious substance that will ruin your life and later try marijuana, only to realize that you were lied to entirely about its effects, you soon break the trust bias between yourself and the "establishment". If they were lying to you about marijuana, heroin can't be
that bad. When the master intentionally deceives the pupil, whether it be a parent, a police officer, or a teacher, problems arise. Genuine advice is regarded as invalid the moment it reaches the pupil, and things start to get out of control. I had a friend who used heroin frequently - he very much enjoyed reasoning as to why he should use heroin, why I should try it, and how everyone would be better off if they just had some heroin and chilled out. He's dead, now... from a heroin overdose. Another victim of the fucked up propaganda machine that is the DEA, that is law enforcement, that is the government in general.
tl;dr fuck DARE, fuck society, and fuck you - I'm going to do as many drugs as I want.