Because it makes you feel good while you're drunk and the way you feel afterward might be worth it. Plus, if you re-hydrate and sober up before going to bed you might feel fine the next morning. That's what usually happens with me.
I have heard it be called "liquid courage." Other times it is to fit in a crowd like at a fraternity/sorority party. That same rule applies in high school too. All the "cool kids" are doing it. Peer pressure.... that's it. As Nom de Plume said, after doing it for so long and adjustments your body makes to break it down, etc., it does become an addiction. There is status and prestige, too. Your daddy did and your mommy and your uncles and all the people on the tv and blah, blah, blah... What some have not mentioned was some of us like the taste. I do. Don does. We don't drink to get drunk. We drink because it is part of a habit we have that allows us mentally to relax. I am sure the alcohol is not part of that. The act of doing it is a mental process. The alcohol isn't doing either of us any favors. That is an entirely different topic.
Feeling sick the next day goes to show it is a poison to your body. You can prevent that feeling however and make it easier on your body by drinking water along they way of your booze intake and before going to bed taking vitamin B, vitamin C, and aspirin. One of two depending on your stomach and physical size.
Alcohol tends to decathect the superego. It's an effective way to shut down mom's and dad's voices. For some "feeling like sh*t" is a small price to pay for a break.
I never felt like crud afterward, because I alternated my alcohol with glasses of water, took tylenol beforehand, and kept hydrated afterward. I take that back. I had only one hangover when I was younger, and it was because I didn't do all of the above. Maybe it was all in my head, but it worked for me, and that was all that counted at the time.
Merlin 