I started studying Japanese because I'm a major language nerd :P I've studied, for various lengths of time, Japanese, Spanish, ASL, Old English, and Hebrew, but like several people have already mentioned, my frist foray into foreign language was Spanish... but I loved it (and still do!) - I studied Spanish for 12 years (1st grade on), but as much as I loved it, by the time I got to high school I just felt like it was too easy, and I wanted to try studying a more difficult language. My high school (and about a dozen other public high schools in my area) had a program where kids could study topics not usually offered at public high schools - there were crazy things like robotics, aviation, pre-law... and for languages they offered Chinese, Japanese, and Russian. Now, my mother is of Russian descent, and she studied Russian in college, so she wanted me to study Russian. However, two of my friends (one of whom was an exchange student from Argentina, the other interestingly enough was also in my Spanish class) were really into anime etc and were going to take Japanese. I thought it would be easier/more fun to take a language with friends so I'd have ready study/conversation partners, and that was that. I was still studying Spanish though, which made things really interesting (there was one time when the word 'battery' appeared on a vocab list in both classes in the same week - pila for Spanish, 電池 for Japanese - and I had the tests on the same day. I couldn't for the *life* of me remember the Spanish - on the first test - and ended up just writing でんち in kana on my Spanish test! Fortunately, my Spanish teacher was awesome - if I could be half as wonderful a teacher as she was, I'll count myself lucky - and upon asking me what the heck I'd written, actually gave me half-credit!)
The more I studied Japanese, the more I fell in love with it, and the more interested I became in the culture, history, etc... and before I knew it, I was an East Asian Studies major in college, lived for three years (well, 2.5) in the Japanese House, and studied abroad at Keio University for a semester. After that semester in Japan I knew I had to go back, and here I am on my 5th (and last! :'( ) year on the JET Programme, and in my 12th year of studying Japanese. Now to complete the circle I'm hoping to become a Japanese teacher in the US, if I can find a job!