" ..the fruit of the Spirit is .. Joy.." Galatians 5:22

" ..God loves not heaviness of spirit; He hates comfortless doctrine, heavy and sorrowful cogitations, and loves cheerful hearts. For therefore has He sent His Son, not to oppress us with heaviness and sorrow, but to cheer up our souls in Him. For this cause the prophets, the Apostles, and Christ Himself exhort us, yes, they command us to rejoice and be glad: "Rejoice greatly, O daughter of Zion; shout, O daughter of Jerusalem: behold, thy King cometh unto thee" (Zechariah 9:9). And in the Psalms it is often said: "Be joyful in the Lord." Paul says: "Rejoice in the Lord always." And Christ says: "Rejoice because your names are written in heaven." Where joy of the Spirit is, there the heart inwardly rejoices through faith in Christ, with full assurance that He is our Savior and our Bishop, and outwardly it expresses this joy with words and gestures. Also, the faithful rejoice when they see that the Gospel spreads abroad, that many are won to the faith, and that the kingdom of Christ is enlarged." -Martin Luther

The joy in the Christian heart is what makes us strong. This joy, which is the Holy Spirit's fruit in our lives, comes from knowing Jesus loves us, died for us, and though we are unworthy in and of ourselves, Jesus has made us worthy, and He now rejoices over us.
What joy should fill our hearts, knowing God the Father, and His Son rejoice over us! This joy is exclusive to those whom have tasted the goodness of the Lord, and who own His mercy, and have by faith embraced His grace and truth. And this joy will not be taken from us, ever, and will in fact grow and become full. That's a promise from our sovereign Lord: John 16:22-24.