Psalm 100, Mizmor L’Soda, is recited during Pesukei D’Zimra. When the Temple stood, we would joyously bring a Korban Toda, Thanksgiving Offering. Nowadays, since we no longer bring offerings, Mizmor L’Soda takes the place of the Korban Toda. Just as the Korban Toda was brought when someone was full of joy, a person should be careful to recite this paragraph with much joy. The Shulchan Aruch mentions a person should sing this with a tune to help bring him to joy.