Psalm 137

Verses 1-4...
    By the waters of Babylon,
        there we sat down and wept,
        when we remembered Zion.
    On the willows there
        we hung up our lyres.
    For there our captors
        required of us songs,
    and our tormentors, mirth, saying,
        “Sing us one of the songs of Zion!”
    How shall we sing the LORD's song
        in a foreign land?

The people sorrowed over not just their captivity but also what what signified by it. They wept over what they had lost. The Babylonian captivity was the means by which God chastened His people for their unfaithfulness. Zion is "the city of God," the place of His dwelling. His favor and blessing had been removed from Jerusalem, and the people mourned this. The harps used for worship were untouched. The captors requested music of them, but the people could hardly desecrate the memory of Zion by using songs of worship to entertain the Babylonians.

Yet on the other hand Paul and Silas sang in prison, during the cold, dark night hours of their captivity. They were not mourning their sinful choices, for they were faithful to their Lord.

O Lord, keep my heart fixed upon You. May I remain faithful and true to You above all else! For then my heart will always be able to sing.