No being in Japan, I hestitated to comment on this, but a couple things came to mind.
In the past months that my circle of friends I see all the time has become almost exclusively Japanese and Korean, my own drinking habits have changed. Though not an alcoholic or binge drinker, in college I could be known to "drink everyone else under the table". But that was in a era where none of us drank to achieve a stupor, it just kept flowing through the evening. As I got older, this tapered off drastically ---- may be a couple of drinks one night a month or less.
But now, its somewhere between the two levels. We always have one or two before dinner, then cap off the night sharing rounds of shots. And if its a night of karaoke/noreabang, the level goes even higher. Reading all this I realized that a carafe of flavored soju goes down pretty quickly these days.
And I think several of you who mentioned the alcohol being a powerful unihibiter are right. It seems to be the whole point behind our consumption. Get over the nerves of stumbling through foreign languages (on both sides!) and to drop the political correctness that often inhibits serious information exchange between cultures. With us its never a contest to see who can consume the most, and in fact we have been diligent about cutting others off when the group feels they have had enough. That's a very different scenario from what we see in the American bars and clubs around town.