People have been celebrating the end of the year and the Winter Solstice way before Constantine claimed that celebration for Christianity. So... not only is the celebration of Christmas ( or whatever you want to call it) traditional among Christians, it's traditional on a very deep level of our collective consciousness. Of course, another equally valid answer would be "Why not celebrate?" Any excuse for a party.
Actually I am very much an atheist, yet I celebrate christmas, and it has nothing to do with getting gifts or the greed as mentioned in some of the above comments, actually I sort of find that judgemental on their part. I celebrate it because my wife and family does, and no, I seldom get gifts, but do give them. It is a family time, which I have off of work and can spend time enjoying those that are important to my and mine. It has nothing to do with getting anything.
Why the heck shouldn't I celebrate at this time of the year? Non-religious people, like myself, and Pagans have been celebrating this time of the year long before JC was even a twinkle in the holy ghost's eye!
This holiday is the Winter Solstice, moved a couple of days for whatever reason... Christmas came along later - MUCH, MUCH later... So stop taking over my holiday.
Darn Christians, don't know nothing! Not even know the origin of their own holidays!