Why does the boy in the Grudge meow?

Answer 1

It has something to do with the cat the family had. When the father discovered that the wife was in love with another man, he murdered the whole family: the cat, the little boy, the mother, and finally himself.

The dad snapped his wife's neck with his bare hands, drowned both the cat and the boy in the bathtub, and then hung himself. So, since the cat and the boy were drowned in the same bathtub, I suppose the evil ghost boy feels a connection to his pet cat (even more of a connection than he felt in life).