If were talking carols not songs then i have 3 main- Hark the herald, the first noel and God bless ye merry gentlemen.
Not a specific reason but they remind me of when i was a child and that brings back very, very happy memories of christmas. Also as there traditional carols they give a real feel to christmas-i like tradition
I can quite happily sit in my house with the TV off and just some carols on at christmas time. Especialy with the dark nights and windy,wet weather-realy gets me in the festive spirit.