There must be a reason why society has agreed that this particular plant must not be taken freely by whoever wishes so. We have touched on the side effects such as psychological damage and lung disease but what is the one real reason? I can`t think of one apart from the harm it would cause youths trying and habituating. Socially disruptive maybe.
If you do some research into the issue there are many reasons why it is illeagal but few of them are scientific (or logical). The original reasonings were influenced by the American Burea of Narcotics in the 20's/30's and are almost entirely political (with later reasoning that took on a decidedly racist tone).
An interesting quote from a 1934 New york newspaper - “Marihuana influences Negroes to look at white people in the eye, step on white men’s shadows and look at a white woman twice.”
And a quote by Harry Anslinger, the head of the Burea of Narcotics at the time - “…the primary reason to outlaw marijuana is its effect on the degenerate races.”
The history of american drug illegalisation is interesting and incredibly illogical. If it weren't for the popularity of alcohol at the time, it could easily be as maligned as marijana now.