I personally find the Puer natus est to be my favorite Introit of the three Masses of Christmas. The music illuminates a text that shows that Christ Jesus, the Son of God, came as a baby boy. The antiphon refers to Him as a Child and as a Son. How can one reject the Christ Child? I don't know how one reject this expression of Divine Love from Our Lord Who sanctified every aspect of human life, even as He is the only human being born into this life to die. But they did, rather quickly, as we see in Herod's slaughter of the Holy Innocents.
Just listen and be absorbed in the text.
Unto us a Child is born, unto us a Son is given. The
insignia of His reign is on His shoulder and His name shall
be the Angel of Great Counsel.
V. Sing unto the Lord a new song, for He has done wonderful
things. Glory be to the Father, etc. Unto us a Child is born...