Today's first reading details the final deportation of the people of Jerusalem and the destruction of the city. And the Responsorial Psalm expresses the anguish of the people as they sit in dejection "by the streams of Babylon."
There's a beautiful parallel structure in that psalm that is very revealing. The people of Babylon wanted to hear the harvest songs, the love songs, the typical songs of the land of Judah. But the Jews (the name itself means "Judean people") just shook their heads. "How can we sing a song of the Lord in a foreign land?" For the Jews, to sing the songs of Zion meant singing the songs "of the Lord." What else was there to sing about for God's people but the Lord himself?