Psalm 136

For His mercy endures forever.

Read chapter 136
"For His mercy endures forever" is repeated in every verse of the twenty-six verses of this psalm.  You can really tell it is written to be sung; I imagine backup singers singing "His love endures forever" during the whole song.  Songs or hymns with a simple chorus usually become most loved by people, which we can imagine this one was probably a favorite especially because it is filled with praise.  
Yet why do you thing this is the chorus, to be reminded over and over again that His mercy endures forever?  Some translation say "His LOVE endures forever," "His LOVINGKINDNESS endures forever," "His FAITHFUL LOVE endures forever," etc.  The Hebrew word is checed and does mean all these different things.  Maybe this was chosen to be the chorus because that's the thing we need to be reminded most about God that we easily forget no matter how simple it seems.  We might not need to be reminded that He's Creator or that He's Just, but when times get tough or scary or change happens what we need to be reminded of most is that God's lovingkindness endures forever, His undeserved favor remains, the Lord's steadfast love never dies.  
In what way do you need to be reminded that "His mercy endures forever"?