It is a mystery.
However, understanding this mystery doesn't have to be a part of getting sober. It wasn't for me. I still get the urge to drink and personally it makes no sense. I feel infinitely better as a sober person and yet the alcohol still calls out my name. I know there are a few reasons why I drink, but it doesn't make it any less insane when I eventually do break down and take that first drink.
It really doesn't matter why it happens. You're an alcoholic so you are compelled to drink. Maybe just leave it at that? Unlocking the code as to why we drink won't change whether we stop or not. There are people out there who have laundry lists as to why they think they drink or why they should stop drinking, but it doesn't help them quit more easily. I knew I should have stopped drinking a year ago when my alcoholism was first rearing its ugly head. Here I am after 18 months of daily drinking still only freshly sober. I have slipped many times, and my sober date has changed many times. Nothing really matters except quitting, and quitting for good.
Keep trying and keep fighting. It doesn't matter why we drink, only that we find a way to stop.