Both genetics and environment. Above all our willing and struggle to be free.
My brother, uncle and cousin have been everywhere between binge drinking to heavy drinking and they consider themselves social drinkers. I used to get drunk occasionaly but after a lot of self-critisism, objective observation and research I admitted my problem, so I desided to quit. Probably all four of us share the same gene, but don't have the same point of view or attitude towards drinking.
So, I will agree with TrixMixer. Some of us may have been born with this loaded gun in our hands but it's a mix of different factors that lead to alcoolism. And don't forget just how addictive and toxic alcohol is. Under certain circumstancies (peer pressure, anxiety, culture, modern lifestyle, role models...) anyone could become addicted.