When it comes to dual diagnosis there are quite a few theories about how it should be dealt with. That is because dealing with someone who has an addiction and a mental health problem at the same time can be very challenging. Furthermore people who have multiple addictions also tend to have multiple mental health problems as well.
One of the first theories is that in order to effectively deal with dual diagnosis the illnesses of alcoholism and mental health must be treated in conjunction with each other. Concurrent illnesses, such as alcoholism and bipolar mania together can become incredibly hard to fix as the treatment will vary so widely between individuals. For instance the medications and counseling you give a depressed person addicted to crack would be different than what you would give a schizophrenic who is addicted to drinking mouthwash.
Another theory is that brain diseases share their biology. A person that is vulnerable to one type of brain disease is also vulnerable to another. Alcoholism and mental illness are both brain diseases that involve the same pathways and chemicals in the brain.
Many drug and alcohol abuse treatment centers really do see the dual diagnosis as sometimes being the result of drug or alcohol abuse. The theory is that the predisposition towards addiction was already there and that the chemical abuse of the brain brought it out.
Another theory about dual diagnosis addicts is that they are using alcohol as a form of self-medication. For instance someone who suffers from depression may use alcohol to give them a bit of a lift. The person who has social anxiety or panic attacks may drink excessively so they can carry on a conversation.
The problem is that taking drugs and alcohol to medicate mental illness only serves to make the problem worse. Prolonged use of alcohol or drugs can lead to a downward spiral of worsening mental illness and alcoholism that can leave the person much worse off than when they started.