When they saw him, they were astonished, and his mother said to him, "Son, why have you treated us this way? Behold, your father and I were anxiously looking for you."
And so he is telling Mary, "Look, this child is set for great blessings to the people, but it's going to pierce as a sword through your own soul." Preparing her for the agony and the pain that she would experience, as she would watch one day this child, hanging on the cross.