I guess two reasons exist; It's just curious as you said, and you are so lovely.

Japan I can't be sure of, but China has a lot of people stare at foreigners, someone even took a video of me on the subway one time. There are a couple of reasons, but I'm not sure which are shared between china and Japan. One is that foreigners seldom come to China, given its reputation as well as western views of its system. Japan may be the same way, but I haven't been there, so I can only speculate. Another is that, since ancient times in Asia, the standard of Beauty has included things like large noses, height, and other things that, by what I promise you is no more than coincidence, common in white or black people. In Asia as well, people feel a great need to learn a second language for economic reasons, which causes a lot of people, most of whom have grown up being taught not to talk to strangers, at all, ever, to debate whether they should practice their English with literally every comefromaway they see. And of course, in Asia it's considered much more reasonable for friends to call on each other for favours, and people will tell you all about this "uniquely Asian" concept of "connections" which in all honesty isn't all that complex of a system, it's literally "friends in high places" times 10, but anyway, with America as a large trade partner of many countries in the east, there's just one more incentive to make international friends. And of course, we stand out. You're not so likely to notice someone's race in, say, America, where there are people of every which background everywhere, and the colour of your skin doesn't do much for your noticability, but in Asia, where the locals make up a vast majority, looking different from everyone else is eye-catching.

It's a western culture and asian culture clash... *or something like that*. When a japanese sees u, they got confused whether they should say "hi" or "domo". So, they would stare and mindset goes " You're our guest!" . It's not a negative thing. :) I live in Malaysia, so if you would visit my country, I would sometimes confused whether to say "western" way or "malaysia" way. It's a Asian thing ... really. :)) Not really a big deal.

To me, it is fascinating something I wouldn't expect. Wish I can speak Japanese.