Well, I think ONE issue is that our culture has turned Christmas into a perfect scenario, like a Norman Rockwell video would be, with smiling Grandparents still married to each other, all generations getting along perfectly, a perfect meal, perfect presents, peace on earth, etc.

And nobody can measure up to the "perfect" Christmas we often think we ought to be having. There are expectations that are totally unrealistic, and we feed it:

"What? You have to work on CHRISTMAS?" "What? You're family isn't coming for CHRISTMAS?" "What? You're going to be alone at CHRISTMAS?" "What? He's have surgery at CHRISTMAS?" and so on. So we just magnify the emotional impact.

That stereotyped version of Christmas is hard on so many people.