While I can’t answer what causes people to drink too much (since I don’t really know), I think what alcohol does is amplify the personality you’ve already got. If you’re a generally happy, kind person… you’ll be a happy, kind drunk, too, just more-so (think: I LOVE YOU, MAN!). If you’re a generally angry, cranky person, likewise. Sad people tend to get weepy when drunk. I’ve known several alcoholics in my life, and most of them were either of the happy or sad variety, rather than the angry ones, thank goodness.