Why on earth be glad about exhaustion? 

Because the Christmas season goes beyond earth. To take the words of a favorite (well, one of my favorites anyway) Christmas hymn:

O Holy Night! The stars are brightly shining, 
It is the night of the dear Savior’s birth. 
Long lay the world in sin and error pining. 
’til He appeared and the Soul felt His worth. 
A thrill of hope the weary world rejoices . . .

Most of us know the story about the magnificent star that shone in the East, signifying the holiest night of the year and proclaiming the birth of Our Savior. 

O Holy Night captures what came before. Long lay the word in sin and error pining. The world had been waiting centuries for the Messiah. Waiting while dead in their sins because there was no way to keep the letter of the law and no one to rescue them. They were pining, which means (according to dictionary.com), “yearning deeply,” “suffering with longing.” They were unfulfilled and lost.

til He appeared and the Soul felt His worth.
A thrill of hope, the weary world rejoices . . .

The change is instant. He appears; the people rejoice. Though they were weary from the waiting, the birth of Christ brings unending hope and eternal joy.

We, on this side of the Christmas Story, do not have to wait. Hope and rejoicing are ours for He has come and will come again.