From St. Louis Marie de Monfort
True Devotion to the Blessed Virgin, no. 243.

Loving slaves of Jesus in Mary should hold in high esteem
devotion to Jesus, the Word of God, in the great mystery
of the Incarnation, March 25th, which is the mystery proper
to this devotion [Holy Slavery], because it was inspired by
the Holy Spirit for the following reasons:

a) That we might honor and imitate the wondrous dependence
which God the Son chose to have on Mary, for the glory of His
father and for the redemption of man. This dependence is
revealed especially in this mystery where Jesus becomes a
slave in the womb of His blessed Mother,
depending on her for everything.

b) That we might thank God for the incomparable graces he
has conferred upon Mary and especially that of choosing her
to be His most worthy Mother. This choice was made in the
mystery of the Incarnation. These are the two principal ends
of the slavery of Jesus in Mary.