I realise I'm posting at the tail-end if this thread's lifespan and nobody'll read this far, but damn it I wrote all this so I am posting it.
I've drank at varying levels over the years, from building up a tolerance and drinking about 8 or 9 pints (plus a couple of shots before the bar/nightclub closes) twice every weekend, to quitting drinking completely for about six months for no other reason than getting bored with boozing. I still drink now, both on my own, and with friends, for different reasons at different times.
One of the big reasons I drink when I go out is that I want to lose my inhibitions. This isn't as big a problem now as it was when I was 18, but growing up being bullied and such made me generally frightened of other people. Even when I rationally knew that most people were decent people in the civilised world, I'd get nervous and wouldn't know how to interact with people. Losing inhibitions makes me more chatty and sociable. Don't get me wrong, taking it to extremes can result in you taking off your shirt, holding it in the air and announcing that you are a flagpole [http://www.penny-arcade.com/comic/2001/6/4/], but for a while I sort of used it as a crutch until I became more comfortable with people. I rarely get drunk any more when I go out, but I do have, say, four or five pints if I go out on a Saturday night, and maybe a few gins and a shot or two if the occasion calls for it. That's over the course of 4-5 hours in a pub. Get the bus home, wake up the next day without a hangover, all is well.
When I drink alone, it's a bit less easy to think of why I want to. Sometimes I go to the pub on my own, and I end up feeling kinda disappointed if I meet someone I know there and get talking to them, because I want to sit at the bar, occasionally talk to the bar staff, and have a few beers on my own. Sometimes I'll get a few bottles of beer and sit in my living room, listening to music or going on a murder-spree in the earlier Fallouts. It's fun.
And, for me at least, it slows down my thinking. I don't want to seem like someone who thinks he's really clever, but I do sometimes have trouble with enjoying stuff that doesn't need me to really concentrate, and I get distracted and end up thinking about other things. And as a depressed fat man who is most likely sitting on the couch in his underwear at 8pm watching an old episode of Have I Got News For You that I've seen three or four times already, that's not great. Sometimes I'll have a couple of beers if I'm trying to write something and get distracted easily. I don't buy into the inspiration thing with booze, but it stops me from getting distracted and thinking of something else when I'm supposed to be writing.
Oh, and I did the self-medication thing for a month or so for grief and depression once. Not recommended. Getting drunk at night, then having to get up for work the next day with a hangover and being really tired and irritable and already depressed just made me a horrible person to put up with for a few weeks.
But, honestly, if you don't see any positives from drinking/smoking/using other drugs, then just don't do it. I think the "don't knock it till you try it" thing is kinda silly, and if you think that drunks are obnoxious idiots, potheads are lazy and shiftless and think that Robot Chicken is funny, LSD users are pretentious and go on about how they think in ways that weren't possible without LSD and smokers smell of stale smoke and you decide you don't want to have any of those qualities, then I think that's a valid reason to not use any of those substances.
I dont smoke or do drugs but I do drink and I can Tell you its FUCKING FUN!!! give it a go some time, and you know, get drunk 
