"Can burst Joy's grape against his palate fine;..."
See in text (Ode on Melancholy)
This is an elaborate but effective metaphor. The only people capable of experiencing melancholy are also those who “Can burst Joy’s grape,” the grape here serving as a reference to the revelrous joys of wine. More simply stated, those who feel joy most intensely also feel melancholy, for the two emotions are inseparable. As Keats’s fellow Romantic poet William Blake put it, “The deeper the sorrow, the greater the joy.” This counterintuitive notion is the poem’s key theme.