It is important to note how the Grecian urn that Keats contemplates acts symbolically in the poem. Throughout, it is regarded as a symbol of eternity or what is beyond time. Note how the speaker addresses the urn at the beginning of the poem:
Thou still unravished bride of quietness,
Thou foster child of silence and slow time,
Sylvan historian, who canst thus express
A flowery tale more sweetly than our rhyme...
It is worth analysing these images carefully to see how the urn operates in the poem. It is compared by a metaphor to an "unravished bride of quietness," indicating the way that, although it is so ancient, it is still pure and unsullied by the corrupting influence of time. It is described as a "foster child" of "slow time" and a "Sylvan historian." Clearly what the speaker admires and is seduced by is the way that it represents the eternal through the way that it has not been damaged by time and how it preserves history.
After the descriptions of the pictures of the urn, the final stanza cements the importance of it to the mind of the reaader:
Thou, silent form, dost tease us out of thought
As doth eternity: Cold Pastoral!
When old age shall this generation waste,
Thou shalt remain, in midst of other woe
Than ours, a friend to man...
I love the phrase "dost tease us out of thought / As doth eternity." It is as if viewing the urn is contemplating eternity, which in turn leads to peace or sadness as that causes us to think about the brevity of our lives. Thus ironically, the contemplation of the urn or of art can lead to an experience of the timeless or and at the same time the ephemeral nature of our own earthly existence. The poem can be thus said to have a bittersweet tone as these two contrary reactions are balanced together. Thus the longing for the permanent is expressed alongside the sombre recognition of our transient state.