Charles H. Spurgeon on Psalms 137:4

Many were the sad thoughts which the remembrance of Zion would call up: the privileges they had there enjoyed; the solemn feasts and happy meetings of their tribes to worship there before the Lord; the Temple —"the beautiful house where their fathers had worshipped" —now laid waste. But the one embittering thought that made them indeed heavy at heart, silenced their voices, and unstrung their harps, was the cause of this calamity —their sin. Paul and Silas could sing in a dungeon, but it was not their sin brought them there: and so the saints suffering for the name of Christ could say, "we are exceeding joyful in all our tribulation." There is no real sorrow in any circumstances into which God brings us, or where he leads and goes with us; but where sin is, and suffering is felt to be —not persecution, but —judgment, there is and can be no joy; the soul refuses to be comforted. Israel cannot sing beside the waters of Babylon. —William De Burgh.