I read a study once that suggested over 90% of alcoholics/addicts observed in a controlled psychiatric environment were found to suffer from an underlying personality disorder. In most cases several overlapping (co-morbid) personality disorders. Hence, the concept of the "dry drunk," and persisting problems and instability after eliminating the addictive substance, is attributed 100% to these preexisting disorders.
I don't know about the statistics, but I've read that the majority of alcoholics also suffer from a mental illness (depression and bipolar are most common). Frequently drinking/using is a way of self-medicating. Remember, alcoholism itself is a mental illness.
The problem is that we pick someone like this, we're attracted to them. I learned I was responsible because I pick someone who has major problems and stay with him. If I let someone -- anyone -- abuse me then it's really my fault.