Acceptance of sweet rice dishes in Japan My father once told me a story about some Japanese guests coming over to Europe. On their stay here they were once served a sweet rice dish. The guests were so shocked that rice would be served this way that they outright refused to eat it. In an effort to save the meal they even poured some soy sauce over it which of course completely ruined it.

Now I'm asking myself, is this the common opinion in Japan? I remember a scene from the anime "One Piece" - which is one of the most popular ones - where some guy is giving a girl trouble for making a sweet rice dish, thus I think it must be a rather widespread habit.

Since rice is so very important in Japanese cuisine I would have guessed that they would use all kinds of flavouring available. Is there a special reason behind this? And does this only occur in Japan or other asian countries too?
Mochi is a sticky rice cake that can be served both sweet and savory.  Sweetened rice pudding is not an uncommon dessert in Japanese cuisine.  I think your original premise, that sweetened rice is unacceptable in Japanese cuisine is flawed.  Rice is a staple in Japanese cuisine, and is used in numerous ways.  Sweet preparations are certainly common.  Perhaps your father's story, and your anime example, illustrate better what might happen when people expect one thing, and are presented with another.  However, I think your generalized conclusion is incorrect.