Albert Barnes Commentary Psalms 100:2

"Serve Jehovah with gladness: Come before his presence with singing." — Psalms 100:2 (ASV)

Serve the Lord with gladness — That is, in your worship, and in all your acts of obedience. Let there be joy in this service. Let it not be with the fear of slaves; not as a matter of compulsion and force; not with reluctance, moroseness, or gloom. Let it be a cheerful, happy service; let it be freely rendered; let it be an occasion of joy to the soul. The service of God is a source of the highest joy that man knows.

Come before his presence with singing — As expressive of joy. So the birds sing; so nature rejoices; so should man—intelligent, redeemed, immortal man—be joyful.