I have heard it said that when someone starts following Jesus, they begin to experience at the same time greater joy and sorrow than ever before. This is because our eyes are opened more and more to what God sees in sin, and in redemption.
Although we all have experiences of happiness at various times during our days, the experience of joy is much deeper and more powerful than happiness. Joy will bring someone through the most challenging circumstances with a sense of determination and purpose. This is how Jesus withstood the agony of the cross:
Let us run with endurance the race God has set before us. We do this by keeping our eyes on Jesus, the champion who initiates and perfects our faith. Because of the joy awaiting him, he endured the cross, disregarding its shame. Now he is seated in the place of honor beside God’s throne.Hebrews 12:1b-2 (NLT)
Joy is a source of endurance as we stay focused on the purpose for which God has created us: to demonstrate the glory of God through living in fullness (peace), hope, joy, and love.