These two reasons often get overlooked, using marijuana can often bring out mental diseases such as anxiety and paranoia to those who are predisposed. If you are prone to anxiety, depression, schizophrenia, etc. marijuana use may amplify the severity or speed up development in some people.
I am not trying to demonize weed, but I am just trying to bring awareness to the things that can come with using marijuana. I have friends that can't even make it through the day without smoking at least an eighth a day, and get psychological withdrawal symptoms without it.