It's quite a long answer so I'll try to be brief.
After graduating from college and then working for two years in a call center, I just basically got sick of my routine and lifestyle. I was blackballed at work which was kind of a sign that I had to get out. I also had a few friends who were living in Japan at the time. I took a trip over there (here) on vacation for two weeks in May 1999 to visit them and I was just blown away at how exciting big city life was (compared to Minneapolis, haha) and how my friends were getting along and having a good time meeting new people and communicating in Japanese (we all studied Japanese in high school and then college). After returning from that trip, I was really pumped about the thought of living in Japan but still had a few doubts. It took me a month or so to get over those and I began applying for teaching jobs. I got an offer in July and was back in Japan in October 1999.
I am half Thai actually, but my family has had a connection to Japan as my parents lived here for a few years a long time ago and we used to have Japanese students and businessmen homestay with us from time to time. Also, because I had studied the language, it was also a motivator to live in Japan to try and further that skill.
If you have any other specific questions, I'd be more than happy to go into more detail. Have you been here or thought about living here?