Robert, your comet theory is clearly the best explanation, I don't know how we haven't thought about it before! There is something so godly and at the same time so low about drinking (mainly wine, I'd say, and Dionysus would agree with me; though I guess I could tempt him to a glass of Vintage Port). You can do it with elegance and style, and the next minute (the one glass too much) you are making a fool of yourself or (the too many glasses too many) you are flushing your guts and your dignity down the toilet. A delicate balance, an art almost, the ability to reach exactly that level of drunkenness which suits you that particular evening.
I've seen people drink systematically and literally have two different personalities of equal importance. If you let me generalize, I'll say that the British drink in order to be able to talk about themselves (otherwise the inhibition is too strong) and communicate their feelings. And also because it is part of their culture (Withnail and I said it all). In Italy drinking is very much connected to having a good meal, bringing together two of the greatest pleasures in life. An italian proverb states that the one who doesn't drink in society is either an enemy or a spy.
So drinking makes us sociable, more tolerant of other people, more ready to see connections and to transform trivialities into amazing discoveries. It also makes us outcasts if we do not indulge: have you ever tried to be a non drinker in Bavaria? You won't have a social life, period.
Personally I like drinking, but only when I manage to reach that precise state of suspension between still knowing what's going on and allowing myself some liberties. Recently I got very drunk on the last day of a mathematical conference. I had a long conversation with a professor based only, if you'll excuse my french, on the word "fuck" and its variations. It seemed very important at that moment to stress my arguments with adjectives and adverbs from the fuck-patois. I'm sure the professor was seeing the point too. Now, this exercise may be a bit dangerous, as you can surely see, but I am convinced that there is a way of charming your audience and dragging them into your state of mind (even if they are not as drunk as you are) without them noticing too much how altered this state of mind is: as I said, it's an art, it requires the ability of controlling your loss of control, I find it challenging and very interesting.
On the other hand, as you say, why can't be as free and happy without the aid of the bottle? Isn't it a bit sad that we like it so much to live, though for just a few hours, in this parallel world of peace and love? It's a big issue for me. I don't do drugs and hate any kind of addiction, I claim to be against all big escapes from reality (not like reading a book, but more like playing computer games 20 hours a day: I've done it for some time and I was very miserable). But I still enjoy and indulge in this occasional merry escape from reality which 4 (or more) glasses of red wine provide. Is this a contradiction?
I will also add that it's not always a family thing. I come from a family of non drinkers (they're not against it, they just don't like it), I don't feel particularly rebellious, it's more a matter of taste. I'm also particularly resilient to the pressure of social persuasion. But I'll join Robert in finding that positive things can come out from letting (moderately) go every now and then. Lechaim!