To put it simply, it's an ego defense. It intoxicates the person so they don't have to deal with their hidden pain. As recluse said, he has a lot of insecurities. Those insecurities are his inner pain.
Lots of other ways to alleviate pain too. I don't drink alcohol, but I have an insipid tendency to fantasize about being invulnerable. In my head, I vicariously play the role of powerful people in my head (athletes, doctors, etc) OVER AND OVER again so I can tap into that sense of effortless power. I also imagine huge crowds of people clapping and praising me after the occasion. I used to do this for hours at a time (especially after feeling powerless due to an argument, etc). It's not as physically damaging as alcohol, but it can be just as psychologically damaging because it involves forgetting/disowning my vulnerability for the sake of a high. Meanwhile, my vulnerability is never actually fixed so I have to fantasize some more. It's an addictive repetitive (and shameful) cycle. I don't like to tell people about this side of me.