Why Christmas Has To Be Merry?

My iGoogle's x'mas countdown timer is showing 27 days to Christmas Day.

"Why is Christmas merry?" I suddenly ponder the status quo with interest.

Personally, is it because all the family is together for a meal and it represents a time of giving, love, or togetherness? Seasonally, is it because we get holidays and we have an opportunity to give and receive gifts or is it because of the good mood that people in general exhibit? Economically, is it because of the boost that results in retailers finally making it into the black? Religiously, is it to do with a baby in a manger?

Anyhow, Christmas this year will definitely mean something different, not that I want it, but it is surely putting things in perspective for me.