Many reasons...

Well, I admit it mainly started with manga, I want to read and understand it, then it was added to anime, movies, actual books, newspapers, anything! I've allways loved languages, so I figured, why not Japanese? I have Japanese penpals so it would also be good to talk to them, and considering I would like to travel to Japan someday then it would also be good to learn Japanese because of that Plus I love all culture-related issues and history, and I like to learn history by the main language of the country it happened if possible (Yes, I'm odd ). So as you can see... many reasons.
I started Korean for the same reasons, never got too far though