For as the crackling of thorns under a pot, so is the laughter of the fool: this also is vanity. Ecclesiastes 7:6 (KJV)
Pleasure in the ‘laughter of the fool’ is vain because it adds nothing substantial to the state of our existence. Qoheleth likens it to the crackling of thorns under a pot, “which makes a great noise and a great blaze, for a little while, but is gone presently, scatters its ashes, and contributes scarcely any thing to the production of a boiling heat, for that requires a constant fire! The laughter of a fool is noisy and flashy, and is not an instance of true joy. This is also vanity; it deceives men to their destruction, for the end of that mirth is heaviness.” (Henry)
Reference:
Henry, M. (1706). Commentary on the Whole Bible, Complete, Eccl. 7:1-6