11 It pleased Christ to take upon Himself the Son of Man, and to become thereby our Brother, in order that His mercy to us might be shown most openly; for "it behooved him in all things to be made like unto his brethren that he might become a merciful and faithful high priest before God."(2) Likewise because Mary was chosen to be the Mother of Christ, our Lord and our Brother, the unique prerogative was given her above all other mothers to show her mercy to us and to pour it out upon us. Besides, as we are indebted to Christ for sharing in some way with us the right, which is peculiarly His own, of calling God our Father and possessing Him as such, we are in like manner indebted to Him for His loving generosity in sharing with us the right to call Mary our Mother and to cherish her as such.
Source: Magnae Dei Matris (Vatican.va)