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I'm not sure about the origin of the custom, but perhaps the action of taking something out of someone's hand feels too close to taking it forcefully from them - i.e. for the same reason it is considered impolite for two people to hold the same piece of food by chopstick (for example, if passing food between people from chopstick to chopstick).
Isn't the chopstick thing related to funeral rituals?
> The relatives pick the bones out of the ashes and transfer them to the urn using large chopsticks or metal picks, two relatives sometimes holding the same bone at the same time with their chopsticks (or, according to some sources, passing the bones from chopsticks to chopsticks). Known as kotsuage (骨揚げ?), this is the only time in Japan when it is proper for two people to hold the same item at the same time with chopsticks. At all other times, holding anything with chopsticks by two people at the same time, or passing an item from chopsticks to chopsticks, will remind all bystanders of the funeral of a close relative and is considered to be a major social faux pas.
Isn't the chopstick thing related to funeral rituals?
http://en.wikipedia.org/wiki/Japanese_funeral
> The relatives pick the bones out of the ashes and transfer them to the urn using large chopsticks or metal picks, two relatives sometimes holding the same bone at the same time with their chopsticks (or, according to some sources, passing the bones from chopsticks to chopsticks). Known as kotsuage (骨揚げ?), this is the only time in Japan when it is proper for two people to hold the same item at the same time with chopsticks. At all other times, holding anything with chopsticks by two people at the same time, or passing an item from chopsticks to chopsticks, will remind all bystanders of the funeral of a close relative and is considered to be a major social faux pas.