We would say that if God is concerned about our holiness (and Heis), then He must also be concerned about ourhappiness--not the shallow, superficial type of happiness, but thehappiness that consists of a deep-seated satisfaction in Him. Thereason is that this type of happiness is necessary tobeing holy; it is a central aspect of holiness. Would God behonored by someone who said "I seek to obey God, but I don't likeit"? Or, "I'm a Christian, but it doesn't make me glad to know Ihave a relationship with God"? God is honored when we follow Himbecause we want to, not just because we ought to.
We are continually taking delight in earthly things; shouldn'twe take even more delight in God to prove that He is more valuablethan everything we have in this world? How is it holy to takedelight in things like friends, family, food, nice weather, and soforth (and there is certainly a place for such delight), but nottake even more delight in the One who is far more important thanall those things?
There are many biblical texts that command us to be happy inGod. If we are commanded by God to take delight in Him,then such happiness is necessary to a holy (obedient) life.
"Rejoice in the Lord, you righteous, and give thanks at the remembrance of His holy name" (Psalm 97:12).
"Love the Lord your God with all your heart..." (Matt. 22:37).
"Shout joyfully to God, all the earth; sing the glory of His name; make His praise glorious. Say to God, 'How awesome are Thy works!'" (Psalm 66:1, 2).
"Delight yourself in the Lord" (Psalm 37:4).
"Rejoice in the Lord always; again I will say, rejoice!" (Philippians 4:4).
John Piper, "True Christianity: Inexpressible Joy in the Invisible Christ."