Matthew Henry on Psalms 137:1-4

Their enemies had carried the Jews captive from their own land. To complete their woes, they insulted over them; they required of them mirth and a song. This was very barbarous; also profane, for no songs would serve but the songs of Zion. Scoffers are not to be compiled with. They do not say, How shall we sing, when we are so much in sorrow? but, It is the Lord's song, therefore we dare not sing it among idolaters.