The psalmist, who is unknown, encourages the people of God to sing and shout to the Lord and reminds them of why they should. He tells them that they should give thanks to the Lord. The rationale that he gives is solid and lasting. Then, in similar fashion, readers are given a solid, lasting rationale for why their thanksgiving should be unceasing.
So, why should we give thanks? We should give thanks because He's good and His goodness is unchanging. He never stops loving us. The Hebrew word hesed, translated “lovingkindness”, appears as a characteristic of God over 120 times. Not only is God both good and loving, but He is also faithful. This psalm prompts worship with the truths of God’s person and deeds.
The New Testament equivalent to Old Testament love and faithfulness is grace and truth, seen most clearly in Jesus Christ.
John 1:17-18 NASB For the Law was given through Moses; grace and truth were realized through Jesus Christ. No one has seen God at any time; the only begotten God who is in the bosom of the Father, He has explained Him.