Disclaimer: NCIS belongs to CBS and Donald P. Bellisario.
He doesn't like Christmas. He doesn't like everyone else being so joyful, so happy, when he isn't.
Christmas used to be good. It was happy, fun, joyous. But that was when everything was going right in his life. That was when they were still there.
Now everything is going wrong. Now they aren't there anymore. And he has no one to spend Christmas with. Well, there's always his good friend, Jack Daniels.
He stands in front of the window, looks at his reflection, and raises his flask in a toast,
"Merry Christmas, Jethro."