Because it goes much deeper than physical dependence. Like smoking it acts as a screen against painful emotions and experiences. As a qualified and experienced counsellor and hypnotherapist I have to do a lot of work with people with these behaviours to deal with the underlying causes to enable any change to be permanent rather than just change substances or behaviours.
Ian - Pagan Editor
"We can never obtain peace in the outer world until we make peace with ourselves."
"With realization of one's own potential and self-confidence in one's ability, one can build a better world. "
The only reason I can think of is that there's a genetic link that can be passed down from generation to generation, kind of like a cancer. So if you happen to "trigger" that genetic, it's beyond a good deal of what you can mentally get past...