While there are many references made to taxation, etc., marijuana has over 400 toxins -- and more carinogens than tobacco cigarettes. That's a pretty good reason in and of itself. Also, "a number of studies have shown an association between chronic marijuana use and increased rates of anxiety, depression, suicidal ideation, and schizophrenia." source: http://drugabuse.gov/infofacts/marijuana.html
The same source also lists other adverse effects of marijuana use.