There are multiple potential reasons for comorbidity. One possible mechanism is that one disorder causes another. It is also possible that some underlying environmental risk factor increases the likelihood of developing both disorders. For example, traumatic events such as severe abuse can increase the risk of developing multiple disorders such as alcohol dependence, post-traumatic stress disorder (PTSD), and major depressive disorder. Another possible explanation for the increased comorbidity is that a common underlying genetic risk leads to both illnesses.
