I think a lot of drug abuse stems from the fact that using drugs is a rebellion in and of itself. People who tend to not trust authority to begin with are largely the same people who take these potentially damaging substances, and I think that has something to do with the fact that they don't trust the medical and social establishment enough to believe that drugs have negative consequences that are as pronounced as is constantly claimed. And there's a good reason they don't: a lot of the information people are exposed to are outright lies (based on old cultural notions and bunk science that has since been discredited). Marijuana is no where near as dangerous or harmful as authority figures constantly parrot to our youth, and the moment they try it for themselves and see, that the trust is broken.
I for one think it seems incredibly suspicious from the perspective of a child who is well versed in history that lots of substances that were legal not too long ago (cocaine for example was made illegal in 1970) and widely used are now regarded as instant death and addiction chemicals. Add on top of that the fact that it is illegal to see for yourself if cocaine is as damaging as is often claimed, I think drug policy in the United States is practically conspiratorial from some perspectives (not my own, nor do I think this is true objectively, but I can see how some people may feel that way).
I really do believe that if we legalized drugs and told people the straight up truth about what happens with them the rate of use would probably climb for the safer substances, but the problem of the harder substances would largely solve itself.
EDIT: To Mystic's point, I don't disagree that Marijuana is harmful to one's health, but so is alcohol, and that's completely legal. Overeating is harmful to one's health and the state of being overweight probably has negative psychological associations, that doesn't mean we should control behavior in those people. I just don't see how a free society has any business controlling behaviors that do not hurt anyone but the individual partaking in them.