Marijuana is a green plant. Both the male and the female varieties are quite useful for a wide range of uses. Needless to say that, for recreational purposes, we all like females better.
And so it happens that its effects are as simple as they are complexly strange. Your body will dwell in space, and your mind. . . well, it is what marijuana makes to your mind what makes it an illegal drug.
Unlike alcohol or tobacco, marijuana has got some astoundingly beneficial effects to your physical health—at the cost of messing with your mind a little bit. It can help you with some illnesses—specifically the chronic, uncomfortable ones—and it can also help you with bad feelings and emotions.
What it does is it relaxes you—and here is where the mind starts getting messed up. Long-term marijuana users get so accustomed to the drug's help in getting them relaxed, that the mind starts to wonder whether she is of any use in the body she lives. An obsessive mind will get obsessed with the idea of the meaning of social usefulness. A bipolar mind will get random onsets of happiness and depression every time the drug is not in the body. A quiet, mindful and introspective mind will get embedded upon itself.
So, unlike other legal drugs available without prescription, marijuana does some unexpected tricks on its hard-bone users. The main thing about marijuana's illegality, when compared with its hated cousin, alcohol, is that it sometimes has an opposite effect.
What alcohol does to the brain is it turns its main social self-control circuits off (or makes them dull); marijuana makes them vivid. Alcohol reduces or impairs inhibitions; marijuana enhances them.
A lot of people may say that the laws underlying alcohol's or cigarette's legality and marijuana's illegality are absurd and arbitrary, but even our most absurd and arbitrary decisions are guided by unconscious factors. Are there unconscious factors underlying marijuana's illegality? I think there are, and I think there are as many good reasons for the drug to remain illegal as there are for it to get legal. But it is not legal, nowhere in the world it is absolutely legal, not as alcohol, cigarettes or aspirins are. Why? Well, I think that it is, in a sentence, because it makes social interaction harder than it already is—but I probably am still too messed up to answer.