Note to King Lear, 1.4.175-176: "Then they for sudden joy did weep / And I for sorrow sung"

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King Lear,
Act 1, Scene 4, line 175

These two lines parody the opening stanza of the "Ballad of John Carelesse." John Careless was a legendary Christian martyr who sang for joy when he was about to be executed and thrown upon a dunghill. The lesson of the ballad is that true Christians, faced with the prospect of martyrdom, can find joy in their faith, and profound meaning in the sacrifice of their lives.

In contrast, the Fool's parody makes the point that Lear's sacrifice of his kingdom to his daughters was childishly stupid.

Here's the opening stanza of the "Ballad of John Carelesse" :
Some men for sudden joy do weep,
And some in sorrow sing:
When that they lie in danger deep,
To put away mourning.