I'd say it's a mix of genes, environment, and simply repeated ingestion of an addictive drug. Some people seem f*cked from day one, others drink for decades before showing a problem. Growing up, my adoptive parents and family drank rarely and never to drunkenness. I was never abused or experienced childhood trauma. But basically as soon as I started drinking in High School I would get wasted on most drinking occasions and feel drawn to drinking every weekend. No idea what my biological parents were like, but considering they gave me up for adoption I wouldn't be surprised if they were a mess.