I do have an ulterior motive for asking this question: Many of the characters in the game I'm working on will be foreigners in Japan.
Anyway, I'll start the ball rolling.
Short version:
I had been thinking about learning another language for a while, and it made sense to learn Chinese.
The problem was, Chinese is almost impossible to learn just from books, and I gave up after a few months.
I thought for a while about another language to learn.
One day, while I was playing Tekken 3, I heard Jin Kazama speak in English and Japanese, and he sounded insanely cool in both languages. So cool, that I decided to learn Japanese.
About 6 months after that, I moved to London, and while I was there, I met some Japanese people, and did language exchanges. I really loved doing this. I learnt a lot, and met a lot of really nice people. Many of them became good friends, and introduced me to their other Japanese friends. I just thought that Japanese people were so wonderful and friendly, I absolutely had to go to Japan, so I arranged for a working holiday visa. So many (great) things happened since then, and I've been in Japan since August 2008, and plan to stay here permanently.