Lament of the exiles

Psalm 137:1-9
By the rivers of Babylon we sat and wept  when we remembered Zion.
There on the poplars we hung our harps, for there our captors asked us for songs, our tormentors demanded songs of joy; they said, “Sing us one of the songs of Zion!
How can we sing the songs of the Lord while in a foreign land?
If I forget you, Jerusalem, may my right hand forget its skill.
Remember, Lord, what the Edomites did on the day Jerusalem fell.
Lament of the exiles
Sing us one of your songs, the captors demand. But how can the exiles sing to God in the foreign land of Babylon? The old joyful melodies bring a lump to the throat. 
Memories of the terrible destruction of their city and Temple overwhelm them. The pain of it comes out in a cry for vengeance: tit-for-tat, 'an eye for an eye'
Why?
Sing one of the songs of Zion, Israelites remember what their situation when Jerusalem fell, they felt sad and they could not sing songs of Zion.
It is the mood of Israelites frustrated, and could not sing songs of joy.
Friends with the world means enemy with Christ, friends with Christs means enemy with worldly.
How?
Keep close relationship with Jesus Christ.