The birth of Jesus opened an eternal portal to life that can not be shut. Jesus fully human and fully God – who is also the last Adam, restores the dead to life. This is the reason for celebration; through the incarnate Christ we see who we were created to be and Who God truly is.
“Thus it is written, The first man Adam became a living being (an individual personality); the last Adam (Christ) became a life-giving Spirit [restoring the dead to life]. [Gen. 2:7.]” 1 Corinthians 15:45 AMPC