With all due respect to my Christian friends, I have to admit I take offense when they want to brag about Jesus being the "reason for the season." If they would indulge me for a moment, I would ask them to close their eyes and follow me as I take them on a little inward journey.
Imagine you are a person living in ancient times, just beginning to understand a bit about the strange world in which you find yourself. You have noticed the great shining one in the sky is a benevolent being. It provides light, and it provides warmth, and it seems to speak to the land, calling forth life in the spring and coaxing the crops to grow and thrive. Then, imagine that you notice this benevolent force shows up every day, and you become dependent on each day's arrival as a positive sign that all is right with the world. You begin to build your perception of the "cosmos" around the idea that the benevolent being, which you have named The Sun, will come back every day...without fail. And yet, there is something disquieting which you begin to notice. You notice the Sun comes later and later each day, and doesn't stay as long. And the Land...whom you have called The Earth and look to as if she were your mother, misses the Sun. The Trees miss this Sun...and they drop their leaves. The air turns colder, and the skies drop snow and cold rain. Lakes and ponds begin to freeze over, and you have to put on more skins to stay warm. Most of all, you notice the darkness begins to encroach on the world. The dark is mysterious, hidden, secretive, and filled with the unknown. You know longer feel as secure about The Cosmos as you once did.
Then, one day, you become aware the days are getting longer, the weather getting warmer, and live begins to return to the green and growing things around you. Eventually you realize that like the cycles of birth, growth, life, death, rebirth...the Sun follows a similar cycle in relationship to these events. So, you prospose the Sun will return each year, and you begin to note and celebrate this event. You bring the greens still growing in the forest into the hall, and you gather the community together with you there around the fire. You share with each other the new skins you have prepared for keeping warm, and you fill the hall with as much light as you can to drive away the mysterious darkness. You sing and eat and drink and make merry to warm your hearts, and lighten your spirits, because otherwise you fall into quarrels and arguments in the close quarters of the hall, and this serves no one. You wait out the Winter, and then you move back out into world when the Spring thaw comes. You call this observance by a number of names, depending on your community, but it is how you make it through the dark, cold winter. And every community does it.
So...this is the reason for Christmas/Yule/Alban Arthan/Kwanzaa/Winterfest. As humans it is only natural we do what we do at this time of year, depending on our local climate. Each tradition can tell us "a" reason for the season as they recognize it. No tradition can really tell us "the" reason for the season...the season is the reason all to itself. Today, if we did not have something like "yule," we would probably have to invent it!