Though Sanrio still hasn't made an official announcement about it, Yuko Yamaguchi, the current official designer of Hello Kitty entertained this question when Time Magazine interviewed her:
"It's so that people who look at her can project their own feelings onto her face, because she has an expressionless face. Kitty looks happy when people are happy. She looks sad when they are sad. For this psychological reason, we thought she shouldn't be tied to any emotion - and that's why she doesn't have a mouth."
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