If you read the book closely, you see throughout the book that Michael was the son who was most like his father.
It says in a couple of places that Michael's expression takes on an uncanny similarity to Vito's.
My favorite Vito-Michael symmetry is one that was left out of the movie for reasons I can't fathom:
Michael: "If a bolt of lightning hit a friend of his the old man would take it personal...."
Vito: "If my son is struck by a bolt of lightning I will blame some of the people here."
I guess they had to take out Michael's line for the movie because they also took out the part where Michael says it's all personal. I don't understand why they did that, either.
