The Choice of Word

[makingmoni]

One day, John was back home after work.
He found that his wife was shaking their daughter who was only half a year old.
She said Da-Dy to the baby many times.
John felt very happy because he thought his wife chose the word Dady to teach their baby.
During one night several weeks later, John and his wife were waken up by the cry Dady.
His wife said to him, Darling, she is calling you.
Then she turned to sleep.

[joyroos18]

why was he not upset she was shaking the baby ?