I remember reading years ago, in NewScientist.com (they used to have a whole section dedicated to alcohol and marijuana studies) that there is a chemical in the brain (I'm sorry, I don't recall the name of the chemical, it wasn't common, and Newscientist.com is these days subscriber based) that was necessary for rational thought. Schizophrenics has a lower level of this chemical than normal. There are people out there who are not schizophrenic but don't quite have the normal level of this chemical, and Cannabis lowers the level of this chemical - sometimes to the schizophrenic level.
Now you can think as you like that nothing can positively prove a connection, but you can go back and see my example of the two brothers, and their story, and many others, pretty much match the above study.
There are also studies on genes and how they affect the likelihood of becoming addicted to any particular pleasurable substance, with the incidents of much higher in people with a certain gene, than those without (yeah, I know, don't have a reference for this one either)
Drugs also affect different people differently. For example, my younger brother who was involved in a very serious fall, is pretty much immune to one of the sedatives they gave him (they were giving him two, and in large dose) and he kept waking up while they wanted him to sleep.
The point is, there's appears to be a genetic lottery to the affects of many drugs on us...but whether or not a specific cause can be proved, there are obviously physiological reasons for this (I'm sorry, I don't buy your 'he was completely normal until he smoked marijuana, but he must always have had mentally psychotic problems because, even though shizophrenia is proven to involve chemical imbalances, hence they can usually treat them with other drugs, marijuana itself couldn't possibly trigger it through physiological means.)