For me it was kind of random. I took a super short summer Japanese course... (Half on a whim, half because a guy I liked was taking it...
) And found it extremely easy. The course itself was a course for hopeful Japanese teachers - and I was part of a group of "practice" students.
For a lot of the practicing "teachers", they could barely speak Japanese to begin with, and were usually far from good at teaching. I think over half the course was different variations on teaching a "daily greetings class".
But where it completely failed at actually teaching any real Japanese - I was able to take away the knowledge that I was very very good at remembering and pronouncing Japanese when compared with all the other students. I was picking up and remembering words from when the Japanese native teachers were discussing things amongst themselves. The head of the program knew my name after the first few days and somewhere along the line she started using me as her main demonstration student.
(Years later, on a visit to the university where she now heads the entire East Asian department... She still remembered me and was not at all surprised that I`d reached fluency.)
She showered me with praise, and pushed me to aim for a degree in her department... At that point though, I was years away from university. She gave me a scrap of information that compelled me to actually head to Japan.
Her courses were test-out courses. I could test out of up to two years worth of language and culture classes. This held the potential to cut a university education down by a year or more.
This, combined with the fact that the university fees were time based - not credit based... And that I was incredibly poor and could hardly imagine affording university at all let alone 4 full years of it...
It became a very real incentive to learn Japanese as quickly and cheaply as possible.
In the course, studying to be a teacher, there was a woman who had to have been in her 40s. She apparently went to Japan in her late 30s, stayed for a couple years, and managed to acquire close to fluent Japanese.
I was 15 at the time of the course. I figured that if someone in their late 30s could manage to acquire the language by exposure, surely I could do the same as I was much younger.
It took a year and a half after that to scrape together enough money to buy a ticket and find someone willing to let me stay with them (the mother of a friend). But I did get myself over to Japan, and I was able to pick up the language through exposure. I never did head back and to that university though. I later got a degree in Japan at a normal Japanese university.