“I, I am he who comforts you;And from 2 Thessalonians 2:
who are you that you are afraid of man who dies,
of the son of man who is made like grass?"
Now may our Lord Jesus Christ himself, and God our Father, who loved us and gave us eternal comfort and good hope through grace, comfort your hearts and establish them in every good work and word.Note that it is God himself, with and through the Lord Jesus Christ, who gives objective comfort - the eternal comfort that comes from having hope. And it is to God himself, with and through the Lord Jesus Christ, that Paul turns to ask that the Thessalonian Christians might have that comfort as a present, subjective reality. (And implicitly Isaiah's preaching is doing the same: since it is God who comforts Israel [objectively], let Israel be comforted by God [subjectively]).
Comfort is a very Advent-y word. It carries with it the sense that there is darkness and grief - it isn't joy or celebration, it's the arm around the shoulders when things are tough. It's someone walking alongside you through the hard times. That God does this, both by giving us objective reason to be comforted and by subjectively comforting our hearts - that is glorious.