I reckon people drink/drug to distract themselves from the pain/emptiness inside. I think there's a genetic component for many too. Put them both together and this can lead to alcoholism/drug addiction/insanity.
I'm going to use me as an example. When I was a teen/young adult, I drank alcoholically. Then I married and had kids. My ex husband is/was an A. My ex partner is/was an A. Whilst I was with them, my addiction to them (as an ACoA) overtook any addiction I could have to a substance and I was the 'sober' one. Due to my experiences with A's, I didn't drink and I hated alcohol. Now that I'm on my own and 'partying' with my friends on occasion, I've gone from a non-drinker to bingeing at times. It's like now that I feel 'safe' and now I don't have a diversion/distraction from my own sh*t in the form of an addicted partner, my own issues with substances/depression/anxiety /lack of self worth, are screaming out for me to 'fix' them. When A's talk about being alcoholic before they even started drinking, they're talking about their isms. The restlessness, irritability, depressions, fears and phobias. They talk about not feeling comfortable in their own skin. I can tick all of those boxes. I've had that for a life time. So, whether I pick up the drink or not, my depression/anxiety/PTSD etc is sitting there, below the surface. Now I understand that I've distracted myself from having to feel my own stuff and my own emptiness by filling the void with a person/partner. If this void is not filled with self-love, self acceptance, HP, a spiritual program etc, then substances and other addictions including people with their own addictions will be there banging on the door (my door) to be let in. Just my thoughts.........