Silly joke on Doraemon and Hello Kitty

Doraemon and Hello Kitty were out on the streets one day.

When Doraemon saw Hello Kitty, he waved and greeted her, "Hello Kitty!"

Hello Kitty, however, didn't greet him in return. Why didn't she?



---> (She didn't have a mouth.)



Now, she was feeling pretty bad about appearing rude, so she stitched a mouth on herself and went off to look for Doraemon.

When she saw him, she greeted him enthusiastically.


Doraemon, however, ignored her. Why didn't he acknowledge her greeting?