Well, I think there is a certain behavioral component to predisposition. My mother, much like me at this point always lived in a world of her own, and by nature sought out pleasure. She became an opiate addict. Of course there is more, her body tolerated opiates fairly well, and she wasn't one of those who can't seem to enjoy opiates, nor was she one who was especially sensitive. I think just who you are predisposes some to certain addictions. Someone who is esepcially nervous/neurotic might have a higher chance of developing an addiction to alcohol for example, than someone who is not.
All of this, though, doesn't take into account the most important factor, drugs make us feel good. Now, cocaine or amphetamines might not be all that fun for an especially anxious person, but for someone who needed more concentration and engergy they might be more of an attractive choice for an addiction.
Some of us are a little "off", a little "nutty", a little "blue". Whatever it is, drugs make people feel better, and for some of us who need to feel better (in which ever way) might be predisposed to certain addictions over others depending on who they are, and how they operate, and function.
I am not sure if all of this makes sense, but I just don't buy all the neurotransmitter defficiancy bullshit, sorry just seems fishy to me.