I think there are multiple reasons and they don’t all have to be present or they can all occur at once or any combination in-between.
1. Learned behavior. The person has been told or has witnessed their parents using alcohol to relax during stressful times. Even worse, it is normal to drink daily. I think if drinking is a normal routine it can more easily become abused.
2. Genetic tendency towards addiction.
3. I think some people really get a much better feeling from the alcohol, stronger chemical reaction in the brain, than others. Maybe this is actually part of answer 2.
4. The majority of the alcoholics I know have a lot of underlying pain in their life from childhood. They seem to view life a certain way, have a lot of anger, and feel they were targeted or cheated. Not always true of course, but I see it a lot.
Also, I don’t know if you are including people who go on antidepressants or antianxiety medication? Alcohol is self medicating, but a percentage of people who take prescribed drugs are kind of in the same boat. If I ever went through an accutely traumatic time again, I would ask for Xanax. It helped me through a very bad time years ago. I was really unable to function, and the physical problems I was having were like a torture. I have no idea if alcohol would have helped, but Xanax for a few weeks was a gift. I don’t see how alcohol would have helped actually. Xanax for me was purely to get rid of the physical manifestation of my sadness; while I think people drink to turn off their thoughts and try to rest. Again generalizations. Everyone is different.