Isaiah 66:13–As a mother comforts her child, so will I comfort you; and you will be comforted over Jerusalem.
Even the most grown up of adults needs a mom again when times get rough — we need someone who loves us no matter what, who knows exactly what to say, what to do, and how to make things feel better. Even Jesus, God Incarnate on earth, needed a mother. The one God chose, Mary, stood by Him through his tough times of being rejected by men, even to the end of His life. As He was lifted from the cross she likely received Him into her arms once again, cradling Him just as she had when He was born and needed her loving comfort.
God knows His Son needed the comforting of a mother, as do we.
And so a young woman became His mother that holy night.