Because this day also meant the birth of the sun.
Last Samhain, the God died, but not before impregnating the Goddess. All winter long, she has carried the young God inside Her womb, grieving for Her lost husband and waiting for Her Son to be born. On this night, She finally gives birth and the world rejoices at the sight of the newborn King, the sun.
This was a common theme way before people got the idea of "God-made-man", so it made sense for the "Son of God" to be born at the same time as God Himself. This theme wouldn't have worked so well if the God-Incarnate had been born at Samhain, when the God dies, or at Mabon, the harvest celebration. Solstice also meant that soon, it would be the growing season, soon, it would be warm, and soon there would be plenty. This "soon it will all be better" meaning of Yule was also associated with the God-Incarnate...this God-man will make everything better, but not this instant. We will have to wait awhile, but it'll happen.
What do these savior-oriented myths have to do with a faith that has no need of "saving"? Taken literally, not much. But the forerunners and basic ideas of these stories are what Pagans celebrate, the turning of the Wheel and the rebirth of the God. The cycle is everything.
This is a family holiday for us, so we celebrate alone, just we two. Rather than scripting a high ritual, we use the old traditions of our respective families, each year trying to find the right mix between the G. family and the N. family. We hope to get the mix right before we have children!
This has served us in an unexpected way... by examining and re-creating the things our families have done for generations, we have discovered deeper meaning in what used to be an empty task. We know *why* the G. family eats only a weak potato soup on Yule eve, and *why* the N. family wears a particular color on Yule morning. These traditions aren't Pagan in any way, but that doesn't make them any less meaningful to us.
Who needs a fancy chalice when they have a 92 year old tree ornament?
