There's another Christmas story told here: God on earth in a humble 12-year-old boy. He looks quite ordinary, so how could He be God's Son? The text states that God's favor was upon His only-begotten, so much so that Christ could extend the favor of God to all of mankind. Mary, Joseph, and the extended family were at Jerusalem because of the Passover. The irony: the Passover Lamb was already traveling with them! Jesus followed the law of Moses until the Crucifixion, the sin offering for the whole world. He and His family went up "according to custom" because Christ had to fulfill the Law perfectly.
He stayed behind in Jerusalem because the Temple was there, the dwelling of God's glory. His parents didn't do a head count. The teachers of the Law were rightly impressed with Christ's perfect knowledge of the Word (after all, He'd written it - and, of course, studied it from young childhood). Later He would be consumed with zeal for God's house. It shows our corresponding lack of commitment to the Word in favor of our word.
Pray that godly zeal would consume you - read the Word, guard it, meditate upon it, study it. You have much to understand, as Mary and Joseph did. Be submissive to God as Jesus submitted to His parents. Through His submission He forgave your sin.
And that is what Christmas is all about.