There is a fair bit in the literature about marijuana worsening symptoms of schizophrenia and triggering a first episode but I've never read anything suggesting that marijuana was a cause.
anecdotally, DH and I both work in ED and most of us have looked after people having a psychotic episode triggered by either first use of MJ or unusually heavy usage or from a different source.
Interesting. I smoked pot a little bit in high school/college (not that often -- probably 20-30 times over the course of 2-3 years), and this one time I got it from a different source than usual, smoked way more than usual, and had my first panic attack, triggering panic disorder that lasted for about a year. I was 20 at the time, a very common age for mental illness to present, so maybe something else would have triggered it around the same time anyway, but yeah, for me it was definitely linked to pot usage.
That said, I support the legalization of marijuana. I will never ever smoke it again, and I will encourage my kids not to smoke it, just like I will encourage them not to smoke cigarettes or drink alcohol while underage, but I'd be totally fine with legalizing it and taxing its sale.