As much as I enjoy the notion of building a big bonfire and getting thoroughly hammered in deference to a pagan solstice festival that has subsequently been adopted by the Christian belief system exhibiting an attitude of "if you can't beat 'em, join 'em", I personally can't see three more important reasons for celebrating Christmas.
Christmas is often the only time of the year when all, or many, members of not just an immediate but an extended family are able to sit down together for a shared experience. Certainly, it is the only time of the year when I seem to stop moving long enough to see the majority of my family members all at the same time.
If you come from an abusive or otherwise dysfunctional family situation, it is understandable that you would not wish to return to that. Other than that, though, in the transient, fleeting nature of our time in this world I feel that it is those people around us whom we cherish and respect - be it friends or family - and their ability to impart some element wisdom, knowledge and love upon us that is most important, and in turn our own ability to impart a portion of the same unto the next or a future generation and so create an unbroken chain of shared experience that is far greater than the sum of our individual lives.
As I grow older, and the older generations of family grow frail or pass away, I increasingly feel that it is important to spend time with those around me, as those older family members are generally a mine of personal experience and family history, and you can never ask them those burning questions that occur to you after they have gone.