The proper answer to this, of course, is: "I'm sorry if I've been pensive, dear. I was just reflecting on what a warm, wonderful, thoughtful, caring, intelligent woman you are, and how lucky I am to have met you." This response obviously bears no resemblance to the true answer, which most likely is one of the following:
Baseball.
Football.
How fat you are.
How much prettier she is than you.
How I would spend the insurance money if you died.

(Perhaps the best response to this question was offered by Al Bundy, who once told Peg, "If I wanted you to know what I was thinking, I would be talking to you!")