You'll never find a stranger bunch of people than westerners who seriously study Japanese culture and/or language. I don't mean that in a bad way at all as I count myself in the category, its just 90% of the people you meet just seem like people I could of met in no other walks of life outside of studying Japanese.
Anyway, from my experience, here are some reasons people want to go to Japan, in no particular order;
1. for career/CV enhancement; work experience abroad can be impressive to future employers
2. to experience a totally different lifestyle that is still in a developed, safe environment
3. japanese martial arts. karate etc,
4. Historical interests, particularly samurai and old style etiquette
5. 'kawaii culture'; small, interesting, (mostly useless) objects young girls like
6. 'otaku culture'; anime/manga or video games
7. Fashion/Boybands/Girlbands/Idols. I think I can put those together
8. Want Japanese girlfriend/boyfriend
There are thousands of reasons but the ones above are the most common I've come across.