“Why….” These are the first recorded words of Jesus.
“must be.” Jesus, as the Messiah, “must be” in his Father’s house, where he would learn about his Father.
“in my Father’s house.” This is a common Greek idiom, and does not mean “about my Father’s business” which has been popularized by the KJV. The Greek is en tois tou patros mou (ἐν τοῖς τοῦ πατρός μου), which literally translates as “in the of Father of me,” The phrase is an idiom; it does not make sense literally, and idiomatically it means “in my Father’s house.” At 12 years old, Jesus knows he is the promised Messiah and the Son of God, and he told his parents that he “must be” in his Father’s house, the Temple. He was surprised that they would think he would be anywhere else. This verse shows a little of the workings of the mind of a 12 year old who is the sinless Son of God. As a 12 year old, he is appropriately disconnected from the pain his absence would cause his parents, but as the Son of God he already felt the mission of God working inside him and knew he must be in the Temple, and he was sitting at the feet of the great teachers there listening to them, asking them questions, and learning from them (cp. A. T. Robertson, Word Picturesin the New Testament).