Why the Immaculate Conception?

The Blessed Virgin Mary lived her life in the state in which Adam and Eve lived before their sin. She was as capable of sin as they were; her life, to this extent like ours, was a series of choices between good and bad, self and other, God's will and her own. her glory, for which all generations will call her blessed, is that in every instance she said, "I am your servant. Let it be done to me in accordance with your word." She, who was full of grace, said, "Your will be done, not mine." When she praised God because He had looked on her in her lowliness, she was not feigning humility. She was uniquely aware that it was God's grace, and not her own merit, in virtue of which she had been set apart. And the consciousness of the gap between her humanity and God's power was uniquely acute in her case.

C.S. Lewis remarked somewhere that we are not to imagine that Jesus had an easier time with temptation than we. In fact, he said, Jesus Christ was the only one who ever felt the full strength of temptation, because He was the only one who never gave in to it. He said by way of explanation something like this: "After all, you don't discover the true strength of the German Army by laying down and letting it roll over you; but only by standing up to it and fighting it at every turn." If I might extend (and correct) C.S. Lewis here, I would say that the Virgin Mary is, apart from her Son, the only one who really knew humility, since it was she who, in every instance, chose obedience, who let God's will trump her own, who refused to be duped into trusting in her own resources.

From Why The Immaculate Conception?, by Father Paul Mankowski, S.J. (Recommended reading of the day, from A Little Battalion).