I totally understand that some families use it as an opportunity to get together, which is absolutely fine, but not everyone has that sort of family - for some people, that would an unmitigated horror!
I totally understand that Christmas is brilliant for children because they get presents and children love presents, but not everyone has children nor is a child.
And I can understand the fact that Christians use it a time to celebrate the birth of Jesus, although none but the most fundamental Christian actually believes that Jesus really was born on 25th December in the year, er, zero... That's not important, though - Jesus' birthdate cannot be accurately determined, so Christmas Day is as good as any other. But not everyone is a Christian.
What I resent, though, is the social pressure of 'compulsory' happiness, i.e. the pathetic attitude that if you're in a state of bliss just because it's Christmas then there must be something seriously wrong with you...
When neighbours cross the road deliberately to ask me why I don't have any Christmas lights up in my living room window, I now simply tell them that I'm a Buddhist...