Well, only thing I can say from my experience as a psychiatric nurse, addictions often go hand in hand with other psych 'issues'. Many people use it to self medicate other conditions, especially in the case of schizophrenia and depression. Those suffering from bipolar disorder will use it to try to calm their manic phases, or to come up during their depressed phase... But it's not always the case... just like you can get each of those illnesses separately, you seem to be able to get addiction on it's own too...
I think addiction is probably as personal as the individual, the same way you get mean drunks and lovey-dovey ones and happy ones and so on. Also might explain why some are 'functional' (and yes, read the opinions on that one) while others barely function, some binge drink on weekends and stay sober all week.
I had a recovering addict explain to me that willpower just doesn't cut it... wanting to quit isn't enough... So... Disease? Perhaps... it's a diagnosis in the DSM IV (the psychiatric diagnosis manual)... does that mean anything? Maybe not.
Just thinking out loud I guess.