Something else I heard in yet another documentary is that MJ and other psychoactive substances don't actually get people high, but trigger naturally chemical reactions in the brain, making the brain get 'high' itself.
I practice meditation and am frustrated that I can't think or meditate myself into the same state that drugs can induce. I can certainly get myself to a better state, a calmer or more serene state, a more positive state, but can't seem to induce a sudden euphoria on the level of drugs.
A "runner's high" is nice, but it takes a lot of work to release that amount of endorphins and it still doesn't feel the quite the same as say a high from narcotics.
Drugs are basically keys that open pad locks on your chemical releases and / or the receptors in your brain. The chemicals that cause the sensations are produced by the body, but without the right trigger, it's difficult to produce them "naturally" in the quantities or potency to produce the same level of high (or to open your neural receptors on such a level to receive those chemicals).