From YOUCAT, fact 85:
85: Why is Mary our mother also?
Mary is our mother because Christ the Lord gave her to us as a mother.
“Woman, behold your son! … Behold, your mother!” (Jn 19;26b-27a) The second command, which Jesus spoke from the Cross to John, has always been understood by the Church as an act of entrusting the whole Church to Mary. Thus Mary is our mother, too. We may call upon her and ask her to intercede with God.