It's probably because this is how their schooling has been structured all of their lives. They know that they are expected to work extra hard in school. There is more emphasis placed on school success than there than there is here. They probably have more teachers that are effective teachers, and know how to get the students to learn, as well as enjoy learning. The fact that they enjoy it seems strange to us because we live in a society where education really doesn't get the support and superior status that it gets in China and Japan.
Kids complain about having to go to school all day, and getting too much homework.
Maybe this is because the teacher isn't giving assignments that are intellectually stimulating enough that the kids will enjoy doing them. Maybe we should take a lesson or two from teachers in China and Japan.