What causes cannabis to cause mental health problems for some but not for others?

Cannabis: If someone is genetically at risk for developing depression, anxiety, or even psychosis (i.e. - schizophrenia), marijuana can hasten their onset and/or exacerbate these disorders. However, almost anyone who uses marijuana frequently enough can develop one or more of these disorders, as the brain will try to adjust by semi-permanently producing or absorbing less of certain chemicals.