God brings joy to our lives, not by substitution, but by transformation. He does not promise to remove the pain and hardship we endure. No, this exists, it is real, and it hurts while we endure it.
What he promises us is something that in hindsight will make the pain look inconsequential. He has such joy in store for us that it will overshadow the pain we experience.
His illustration of the woman giving birth makes this clear. Jesus did not say that the mother’s sorrow and pain were replaced by joy, but that it was transformed into joy. The same baby that caused the pain also caused the joy! And so it is in the Christian life: God takes seemingly impossible situations, adds the miracle of His grace, and transforms trial into triumph and sorrow into joy.
That is what he promises to do for us. Although the pain preceding our rebirth into Everlasting Life may be intense, at the moment of or rebirth, all anguish will be forgotten out of the joy that Everlasting Life will bring.
We must continually see life as transformation from the physical to the spiritual. This will help us keep the events of this life in perspective.