Christmas is an accumulator of childish happiness.
Children can be much more happy than adults. Adults have experienced death of dear ones, farewells or illnesses, or realization that some of their goals they wholeheartedly wanted to achieve cannot be achieved anymore in this life. This all remains constantly in the head and cannot make room enough for a full and absolute happiness. Though children do have this room, and therefore can be absolutely happy.
If you as a child were made absolutely happy on every Christmas (or other big holiday in other cultures), this holiday can become a “tag” for that childish happiness, which can be used later in the adult life to remember of those happy times and to try to be happy again.
For me, this happiness tag is triggered by the smell of the christmas tree and the mandarines. Yesterday I was in subway, when I saw one tiny fir tree brach lying on the floor and radiating its smell. And I uncontrollably laughed.