I don't know if this helps but ...
I only just recently quit smoking (as in 2 to 3 months ago). Often I'd quit for a while, but then eventually go back. Usually, it was stress that drove me back. I really honestly felt that I could not function as a normal person without the nicotine. Often, in my bouts of quitting, I'd seek some other substance to "fix" what I felt was broken. It is as Fyrchick stated, a lack of emotional coping skills. Before I recognized it was anything like that, I was constantly justifying the addiction, even to family members who were trying to shame me off the cigarettes.
For what it's worth, it was always to fix my ADD (attention deficit disorder). I was never put on drugs for it growing up, and I eventually turned to self medicating myself to fix my lapses in concentration, poor reflexes, and inability to control frustration. I can't remember the day it dawned on me that this is just who I was, and no amount of drugs would "fix" me or make me normal. I think it was more of a gradual process of making comparisons, and seeing my behaviours exhibited by others, that eventually led me to that conclusion. It wasn't a sudden light bulb turning on kind of a thing. It was just something I eventually knew. Even though it had been stated several times, knowing it in your head, and knowing it in your heart are two different things. The Willpower to quit and turn away for good comes from knowing it in the heart.
Again, don't know if that helps, but there you go.