As for the Gospel text, Rev. Aaron Koch once wrote this (find the rest of the sermon here):
Because Jesus is without sin, He is all the more easily neglected and taken for granted by sinful parents, who travel for an entire day utterly unconcerned about a Boy who has never demanded anything of them or been a problem for them. What Jesus is doing during this three day separation is His Father's business in His Father's house. His Father's business is saving Mary and Joseph and people like you and me. And so when His mother asks Him, "Why have you done this to us?" meaning the worry and the suffering she and Joseph have endured, the 12-year old Jesus replies with words that basically mean, "Because I love you." His Father's business is not to torment Mary and Joseph, but to redeem them. But caught up in their own self-focused perception of things, they didn't understand it. They forgot who Jesus was, even who His Father was, and therefore who they were.
Don't we also sometimes do that? We get so used to Jesus that we forget who He is. We whittle Him down to someone who fits manageably into our plans. And then when He behaves as the Lord of our lives, we get all upset at having to fit into His plans. But Jesus again reveals the truth of who He is to Joseph and Mary and to us that they and we might be called back to repentance and faith in Him who has come to save us. That is His Father's business.