I love that verse, and the more I read it, the more I delight in what it says. First, it tells the people of God (who were chosen by God, see Psalm 135:4-6) to give something to YHWH (JEHOVAH). What are they told to give? They are told, "Give thanks to the LORD."
And for what reason are the chosen people of God (and not them only - Rom. 1:21) commanded to give thanks? It says, "for he is good, for his steadfast love endures forever." The rest of this Psalm continues to show how the Sovereign LORD is good to all His creation and how He has shown steadfast love to His people by saving them and giving them an inheritance.
For these reasons then, we are told to give thanks: First, because the LORD is good. To even know what goodness is, we need to know the LORD. He is good. There is no good apart from Him (Psalm 16:2). God is the very definition of what good is, and so we are to give thanks to Him for His wonderful goodness. Second, we are told to give thanks for His steadfast love endures forever. Other translations of the Bible use the words mercy, kindness or lovingkindness. The Hebrew word translated as steadfast love in the ESV is chesed (Stong's #H2617), which has the meaning of kindness, as well as all those I just mentioned. The point is this, God's kindness, mercy, His steadfast love toward His people never ever ends. What does it say, "for his steadfast love endures forever."
God is eternal, His goodness is eternal, His mercy and kindness and love are eternal; never ending, never changing, never failing. However, we need to remember that because God is good, He must destroy sin and sinners (Rom. 3:5-6). I thought, after reading this psalm, of all the people around the world who are suffering terribly. This is God's goodness and mercy. Why? Because instead of destroying us all immediately for our numerous sins, He has instead been patient, inflicting us with daily reminders that we have sin, we are under judgment, and if we believe the gospel that Jesus Christ is the answer from God the Father for this problem, He will save us forever.
Here is how God shows His steadfast love for such wicked and wretched people who have been chosen for salvation: Jesus unrolled a scroll given to him in the synagogue at Nazareth and read,
"The Spirit of the Lord is upon me, because he has anointed me to proclaim good news to the poor. He has sent me to proclaim liberty to the captives and recovering of sight to the blind, to set at liberty those who are oppressed, to proclaim the year of the Lord's favor."(See Luke 4:17-19). Jesus stepped into a world that hated him, had rejected him, was under the condemnation of the holy law for sin, is totally worthy of destruction by God, and yet He came to set people free from their own deserved guilt and judgment by dying for them on the cross. He came to show the never ending steadfast love of God for wicked, vile sinners. This love is unending if you are one of His chosen, because His resurrected life is unending (Rom. 6:9; Heb. 7:25). And whom does God choose? The poor (poor in spirit - Matt. 5:3), the captives (captive to their sin - John 8:34; Rom. 6:15-22), the blind (spiritually unseeing - John 9:35-41), the oppressed (by the devil - Acts 10:38). In other words, God's unending steadfast love is shown to those who recognize they are completely miserable, helpless, worthless and bankrupt of all righteousness of their own, and so find eternal goodness and kindness in the righteousness of God given solely through Jesus Christ our Lord! (See Rev. 3:17-19)
If you are one of these who have fled to Christ Jesus for help, then "give thanks to the LORD, for he is good, his steadfast love endures forever!" Hallelujah!