As St Bernard so lovingly put it, if he had called her “Mother,” she would have been just His mother and no one else’s. In order to indicate she is now becoming the mother of all men whom He now redeems, He endows her with the title of universal motherhood: “Woman.” Then indicating with a gesture of His head the presence of His beloved disciple, He added: “Behold thy son.” He does not call him John, for if he did, John would only have been the son of Zebedee; He left him unnamed that he might stand for all humanity.

Our Lord was equivalently saying to His mother: “You already have one son and I am He. You cannot have another. All the other sons will be in me and I in him. Hence I say not: ‘Behold another son!’ but ‘Behold Me in John and John in Me.’”

It was a kind of testament. At the Last Supper He willed to mankind His Body and Blood. “This is My body! This is My blood!” Now He is willing His mother: “Behold thy Mother!.”

–Bishop Fulton J. Sheen ‘Seven Words of Jesus and Mary’