1. because the Church has lost Christmas. Let’s face it, the “Christmas” MOST people celebrate –even Christians– is hardly “Christian” at all but is more or less of the same cloth as secular humanism. Christmas is no longer a holy day (i.e., “holiday”) but has rather become a busy season wherein grouchy consumers purchase far too much for far too many people who will in the end care far too little for that which pushes along a far too greedy economy which reduces people made in God’s image to mere consumers of “goods”.
2. because Christ has indeed come. Advent marks the beginning of the liturgical Christian calendar and thereby sets the tenor for the worshipping life of the people of God. Interestingly enough, the Church has seen fit to arrange its worshipping life in a telling way: remembering that Christ has come –in a sense, invaded– our world (N.B. the term adventus [Latin, coming]) and has become a man to redeem humankind.
3. because Christ will indeed come again. The Church likewise begins its year with the sobering reminder that though He came once in humility and to redeem, He will surely come a second time in might and to judge. The Babe of Bethlehem is also the Ruler of the world. The one who came quietly on a Silent Night will return with the sound of a trumpet and receive His own.
While undoubtedly countless other reasons for celebrating Advent could be enumerated and expanded, these seem to me to be the most elementary and –therefore– also a fair and reasonable starting point.