If I remember correctly US servicemen are exterritorial, when in Japan. This means that (as far as I know and I am not a specialist on the matter) courts in Japan cannot pursue legal action onto the american troops.

That can lead to poor behavior within a small part of the contingent (as you can always find an irresponsible person in any community, no matter how ell disciplined) thus creating a somewhat negative image of the american serviceman.

This, coupled with long presence of the troops in Japan and the recent controversial political decisions (Iraq and so on) could make it hard for a person to socialize with the locals.

Also being away from home for extended periods can be difficult, especially in a different culture. The other question is how much leaves you would get, in my country we can be very strict here, so you might not have much free time to spend socializing after all.

And god forbid Japan going to war with China or Russia, while you are stationed there as it would go messy very quickly, or any other event of a major force occurring.