You have loved righteousness and hated wickedness; therefore God, your God, has anointed you with the oil of gladness beyond your companions.” – Hebrews 1:9
His master said to him, ‘Well done, good and faithful servant. You have been faithful over a little; I will set you over much. Enter into the joy of your master.’ – Matthew 25:21
If a lifeguard saves you from the undertow of the Atlantic Ocean, you don’t care if he is gloomy. It doesn’t matter what his mental state is when you are hugging your family on the beach. But with the salvation of Jesus, things are very different. Jesus does not save us for our family, but for himself. If he is gloomy, our salvation will be sad. And that is no great salvation.
Jesus himself—and all that God is for us in him—is our great reward, nothing less. “I am the bread of life. . . . If anyone thirsts, let him come to me” (John 6:35; 7:37). Salvation is not mainly the forgiveness of sins, but mainly the fellowship of Jesus (1 Corinthians 1:9). Forgiveness gets everything out of the way so this can happen. If this fellowship is not all-satisfying, there is no great salvation. If Christ is gloomy, or even calmly stoical, eternity will be a long, long sigh.
But the glory and grace of Jesus is that he is, and always will be, indestructibly happy. I say it is his glory, because gloom is not glorious. And I say it is his grace, because the best thing he has to give us is his joy. “These things I have spoken to you, that my joy may be in you, and that your joy may be full” (John 15:11; see also 17:13). It would not be fully gracious of Jesus simply to increase my joy to its final limit and then leave me short of his. My capacities for joy are very confined. So Christ not only offers himself as the divine object of my joy, but pours his capacity for joy into me, so that I can enjoy him with the very joy of God. This is glory, and this is grace.
– John Piper
Seeking and Savoring Jesus Christ, pages 35-36