The central message of "Ode on a Grecian Urn" is the way in which the Grecian Urn represents an eternal beauty that contrasts to the ephemeral nature of man's existence. This is something of a double-edged sword, as the poem makes clear. On the one hand, the urn acts as a "friend to man," as the speaker says, pointing us towards eternal beauty and making us appreciate the life that we do have more. However, on the other hand, the existence of the urn reminds us simultaneously of how brief our days are and points us inexorably towards the truth of our own lack of longevity. Consider how the end of the poem makes this point:
When old age shall this generation waste,
Thou shalt remain, in midst of other woe
Than ours, a friend to man, to whom thou say'st,
"Beauty is truth, truth beauty," - that is all
Ye know on earth, and all ye need to know.
This paradox, that the urn acts as both a symbol that is loved and hated, does point us towards joy in life as contemplating the urn "teases us out of thought" as we are faced with eternal beauty against the backdrop of an increased understanding of our own mortality.
In "Ode to a Nightingale," a similar use of paradox can be noted. Again, the nightingale acts as a symbol of transcendent and eternal beauty that is above and beyond human experience and outlasts man's brief span of days:
Thou wast not born for death, immortal Bird!
No hungry generations tred thee down;
The voice I hear this passing night was heard
In ancient days by emperor and clown...
Yet out of the sadness of the speaker's state, paradoxically the song of the nightingale is something that gives him great joy and comfort as he is reminded of his mortality.