I think I love Advent because I find it to be one of the more mystical of the seasons...not that Easter isn't the most mystical of all, but somehow the mystery of Advent seems more immediate and accessible.
I guess it's the manifold senses of the arrival of the Lord that we remember especially at this time:
We recall our faith that he was begotten of the Father before all time, a perfect Word spoken forth as a complete expression of the Eternal Source. We remember his birth as one of us, taking our human nature, yours and mine, to himself. We look forward to his coming again in glory in the final fulfillment of his Kingdom.
Finally, we go back into the darkness and quiet of our own selves, seeking the Lord who is struggling to be born in our prayer, in our discernment, in our knowledge, and in our consciousness. Here we find our own Marian vocation, to be those who conceive and bear the Word, that our souls might become Virgines ecclesia factae, Virgins made church, to use St. Francis's language.
To me the mysticism of Advent is the near-overwhelming realization that these are not discrete manifestations of the Son. Though not identical, they are all one.