The reason for the
season, as in the winter holiday one that we are in the midst of, is the season itself. In any society that has a
winter, especially a bad one, you find lots of winter celebrations. These take the form of
Winter Solstice celebrations, completetion of harvest celebrations, or simply "Let's all get drunk" celebrations. The basic idea is to stave of
cabin fever. In societies that don't celebrate winter holiday's in a big way (like the early
Puritans) you get things like the
Salem witch trials. Anyone who tries to tell you that there is any deep spiritual meaning to the holiday's is taking their
dogma a little too much at face value. This is not to denigrate spirituality, using this time of year as a time for reflectiveness on good and joy and peace is a wonderful idea. But it is certainly not the reason for the season.
Interstingly enough, there are many societies in warmer climates that have no winter celebration, especially ones that didn't do much in the way of astronomical measurements (hence they didn't know that of the Solstice).