Why should we become missionaries of Mary?

This does not mean that one who has discovered Mary through a genuine devotion is exempt from crosses and sufferings. Far from it! One is tried even more than others, because Mary, as Mother of the living, gives to all her children splinters of the tree of life, which is the Cross of Jesus. But while meting out crosses to them she gives the grace to bear them with patience, and even with joy. In this way, the crosses she sends to those who trust themselves to her are rather like sweetmeats, i.e. "sweetened" crosses rather than "bitter" ones. If from time to time they do taste the bitterness of the chalice from which we must drink to become proven friends of God, the consolation and joy which their Mother sends in the wake of their sorrows creates in them a strong desire to carry even heavier and still more bitter crosses.

The difficulty, then, is how to arrive at the true knowledge of the most holy Virgin and so find grace in abundance through her. God, as the absolute Master, can give directly what he ordinarily dispenses only through Mary, and it would be rash to deny that he sometimes does so. However, St Thomas assures us that, following the order established by his divine Wisdom, God ordinarily imparts his graces to men through Mary. Therefore, if we wish to go to him, seeking union with him, we must use the same means which he used in coming down from heaven to assume our human nature and to impart his graces to us. That means was a complete dependence on Mary his Mother, which is true devotion to her.

Saint Louis de Montfort (1673-1716), The Secret of Mary