Ave Maria Meditations
The main reason which demanded the Immaculate Conception for Mary was her divine maternity. It was fitting that the Mother of God be always without sin, even original sin. The Son of God could create for Himself a Mother most pure; and He certainly has done so.
His love for His Mother is infinitely greater than ours. If Mary had been conceived in sin, she would have been in a state of enmity toward her Son before He had chosen her as His Mother. Since He had come as Savior to redeem mankind, it was fitting that He redeem His Mother in a more excellent way than the rest of mankind, that is, by preserving her from sin instead of only delivering her from it.
Devotion to Mary Immaculate will lead us to imitate the sinlessness of the Blessed Virgin Mary and her perfect love for Jesus. We should take the Immaculate Virgin as the special Protectress of the purity of our body and soul.
From the complete Marion prayer book