To the gently veiled reproach of Mary, Jesus replies, apparently with wonderment, with another question. It had come upon him so quietly and yet with such irresistible force that the temple of God was his real earthly home, that he marvelled at his mother's slowness of comprehension. Why should she have been surprised at his still lingering in the sacred courts? Did she not know who he was, and whence he came?
As twelve years before, Mary - pondering in her heart - had treasured up the rough adoration of the shepherds and their strange story of what the angels said to them about her Child, as doubtless she had done too when the Magi laid their costly gifts before the Babe at Bethlehem, and when Simeon and Anna in the temple spoke their prophetic utterances over the Infant; so now the mother, in quiet humble faith, stored up again her Son's sayings in her heart, waiting with brave and constant patience for the hour when her God should grant her to see face to face the mysterious things she had hitherto seen in only glimpses.
[Excerpt from the Pulpit Commentary]