A recent question posted as a comment has brought me to talk about a subject that has been on my mind for quite some time but I've been reluctant to approach it. The question was about whether I have any friends who are native English speakers or Japanese. The easy answer is "yes", but not as many of either as I used to have.
Since they are really two different topics, I'll do this in two parts starting with foreign friends. Making friends when you first get off the boat is a little like making friends when you're in elementary school. In most cases, you work some place where you are with a large number of people who are in the same circumstances as you. They are in a new environment which is intriguing and a little scary.
In many cases, you are all also in a new situation regarding work. You will commonly find yourself training with others and you journey through the rocky terrain of learning to work in a foreign country together. It's a wonderful bonding experience at first and you make a lot of good friends this way. It doesn't hurt that, since you're all new to Japan, you're interested in exploring a great deal of what the environment has to offer.
When you first arrive, most of you are all in the same place emotionally, financially, and curiosity-wise. If you're lucky, the people you meet early on will stick around for awhile. It really does start to seem like journeying from childhood to puberty with your friends as you all start to go through the stages of infatuation, disillusionment, depression, and acceptance based on a more realistic view of Japan as time goes by. Those who remain for a half decade or more with you will have a lot in common with you.
The problem starts to occur when you stay behind and your friends move on. If you remain as a teacher, the influx of new recruits will be out of sync with you in regards to interests and emotional reactions to the situation you're all in. If you move from being in a language school to working in an office (as not too few people do including myself), you find your circumstances taking on a different and more complex set of variables that make finding a new network of friends difficult.
As is the case with graduation from high school, those who remain in the home town and those who move out and do other things will grow apart. The people I originally became friends with moved back home a very long time ago. We kept in touch for awhile but it wasn't the same and correspondence either faded away or fell apart.
In my office, I found a wonderful kindred spirit in my Australian boss. We were the only two permanent foreign staff members and became good friends. As I mentioned in my answer to the questions I was asked, we're still friends but getting together has become very difficult logistically. He works from 9:00-7:00 on weekdays. I teach privately from Wednesday to Sunday (in accord with my husband's schedule). Since my private lessons finish in the late afternoon or early evening on Saturday and Sunday, we only can get together on national holidays that fall on a weekday.
Once you leave the environments which facilitate friendships the best, you find that it's very hard to maintain them. I think this is both a part of life everywhere and a part of getting older. As people grow older, they lose the energy to chase down a few hours here and there with friends. This is much more so the case when the involved parties have to rely on public transportation to get around and when they tend to work overtime a lot.
If your work involves a lot of talking, it' so much the harder because you're drained and overtimulated. You want to go home and relax. You don't want to go out to a crowded and noisy restaurant or bar, sit in the secondhand smoke, and be served crappy over-priced bar food just so you can grab a few hours of chat with your friends. I know what it's like because I used to do it a lot in my first five years here. My friends and I are both too old for that now.
For me, my main in-person friendship is with my husband. I know that sounds cliched but my husband is the perfect best friend in addition to his other "bonus" characteristics. Even if I weren't married to him, I'd want to spend time with him talking and sharing my feelings about life. I'll be the first to admit that my desire to go out and find other friends is probably reduced because I'm very happy with him. It's hard to seek a meal when you're not very hungry.
Besides my husband, I speak with one or two friends back home via Skype on an almost daily basis. Ironically, it's logistically easier to have a friendship with people back home than in Japan because the time zone difference allows their free time to coincide with mine much more easily and they work more "normal" jobs.
One thing you realize is that whether you move on or not, your friendships will never be the same. While she maintained a blog about her time in Japan, a blogger named Tokyo Rosa (who has since removed her blog) mentioned that, upon returning to the U.S. after her stay in Japan (which I believe was one year), her friends had moved on with their lives and she didn't immediately have the energy to reinsert herself into their lives. Whether you're gone for a year or 18 years, people move on and apart pretty rapidly and it happens whether you're in another country or just another city. It's just a part of life.