Psalm 100 mentions “joyful” (v. 1), “gladness” and “singing” (v. 2), “thanksgiving”, “praise” and “thankful” (v. 4), because (a) God has made us, (b) we are his people, and (c) his mercy is everlasting (pictured perfectly in Jesus). This is the foundational focus of our lives in relationship to him, and when we keep that front and center, the joy will show up in our lives for all to see.