These combinations are completed in the captors' strange command for their victims to sing a happy song of Zion. The complementary unit then turns into the rhetorical question in verse 4. Most stories in both sacred and secular literature have more than one theme. To write about literature in a convincing way, you need to find themes that you care about and can support from the plot. A theme is not the same as a moral.
Some people have a hard time admitting mistakes and asking for forgiveness.
Sometimes people regret decisions they have made. Sometimes siblings are jealous of each other. Sometimes mothers and daughters don't seem to be visible in the stories. What you do with these themes is up to you. Your goal is to find themes which you can support from the plot and which give you an opportunity to interpret the text in your own way.