(This is what I have learned in culture class, if anyone thinks am I wrong, and by extension my professor, please say so, I am only repeating roughly what he told us)
There are many aspects of everyday Japanese life and culture that Americans, even those going to work in Japan, are never told. Some of these things are very important to know when forming friendships and relationships, both personal and business, with Japanese people.
Firstly, the Japanese are very harsh when it comes to drunk driving. Because of this, Japan has an extremely low drunk driving rate. Many times, if you are arrested for drunk driving it may make it to national news. Getting arrested means you will almost certainly be fired from your job. Beyond the legal blood alcohol level, in normal Japanese situations consumption of any alcohol (even one beer) means you shouldn't drive home. In downtown areas there are many taxis with two drives, one will drive you home in his taxi, and the other will follow you in your car. The Japanese are very concerned with responsibilty and respecting other people, so putting yourself and others in danger when it can be easily avoided is social suicide.
For relationships with other people, making friends carries with it much more responsibility and obligation than in American culture. If you are friends with someone, it commonly brings with it a level of obligation to the other person that in America you may only give to your family and very close friends, if that. Because of this, on average most Japanese have a smaller group of friends than Americans, but their networks can reach much further. Ignoring these aspects of the friendship can cost you a lot some of the time, as that lost friend may spread the word that you are not honorable to other people. Many businessman are hesitant to make many new friends, as it usually means that they will have obligations to use company resources to help the other person if they are in need, when they may not always be able to afford it. In this way, many men depend on their wives to make connections to other wives and thus their husbands for business interests that do not have that long term obligation. (As long as their wife doesn't have a career of her own)
This relationship culture also affects dating strongly. In America, you could meet a person in a bar, or in some public place, go out to dinner and a movie together and then decide if you are interested enough in that person to continue going out. In Japan, you can't just approach some person in public you find attractive and start chatting them up. There are very intricate social circles that exist; it isn't easy to cross the boundaries at will. Usually you may meet a person through someone else, and hang out with them in groups for a long time before even considering going somewhere alone. An 'American style' date would be fairly serious for a Japanese couple. If a couple is frequently going out alone then their families will soon expect that they will become engaged. For Americans, this seems sort of silly since it's precisly those kind of dates where you really get to know a person to tell if you can live the rest of your life with them.
That's all for now, maybe I'll write a little more later. I'm always open for a discussion too if anyone has any comments or questions.