But if I'm not harming anybody, why should I be arrested?
And if we can write laws which cover behavior that is potentially harmful, the why can't we write laws against drug use?"
The difference lies in the property that is being used. If I put a race track on my private property, I can drive as fast as I want, around and around. I can do it while I'm drunk, too. Public property, such as roads and highways, belongs to all of us, so we come to an agreement on rules for what is acceptable behavior on it. Nobody has a right to walk onto my private property while I am driving wildly around my race track, drunk as a skunk, popping wheelies, to order me to stop what I'm doing or face arrest. But, to intervene if I were behaving in such a way on public property is the right thing to do, because I would be violating the laws that we have agreed to.
Are you looking for a government program to help you stop driving like a madman? Ever leave your private property to steal from others to buy gas? Do you even care that your driving around like a madman, with your noise and smell and dust, interferes with your neighbor trying to live his life in peace? Is your yay-hoo driving influencing teens who, lacking their own private property, take that behavior to the public streets? Do you care about that, or is that their fault and they should know better? Maybe it's their parent's fault.
Now, granted, you can bring up that tired analogy of alcohol. Sure, alcohol does those things. But, why do you ask others to legalize other drugs which would add to the problem? I can't think of one good practical reason.
Recreational drugs, like alcohol or even cigarettes, are not restricted to private property. Your analogy sucks is lacking.