Psalm 106 - This is not quite a totally sad chapter, but the Lord shows how often He had to rebuke His people. You may remember the pattern in the Book of Judges - failure - repentance - victory - repeat. Note in this chapter verse 15, God gave the people what they thought they needed but in the process their souls grew lean and weak.
Psalm 107 - Four times over the words are, "Oh, that men would give thanks to the Lord for His goodness..." Find all four times and read between each time why there's a reason for such gratefulness. When God's people believe God's word and do God's work and follow God's way, they have all the reason they need to rejoice in Him.