It's also a typical japanese mindset. Japanese, by culture, don't make friends as easily as western people. It takes a lot of efforts for them to maintain a long distance relationship.
I read an article, here that states that typically, once you don't have anything in common (job, club, school, city), you are not really friend anymore.
Citing the article :
"The logic runs that in Japan it is awkward, untoward, even rude to extend a relationship beyond its “natural shelf life.” This goes even just for moving to another city in Japan: Consider it normal to lose touch with everyone you leave behind. The thread of camaraderie is that thin in Japan."
It's probably the same to some extent for "simple" love relationships, like dating and such.