The Grecian Urn was actually a Greek vase that symbolized Keats' presumptions of eternal life. During this time, Keats was an ill man during the creation of this poem, which in part, influenced him to create this ode. In the beginning of his speech (and of course, one can rightfully assume that it was indeed Keats giving the speech), the speaker's voice is rather uplifted with a flow of light-hearted emotion as he conversed on. All happiness aside, the nature of the ode was actually a bit melancholic with hints of the supernatural world.
A loss of life ideal is evident in this reading. He talks about the urn’s immortality and the pictures on the urn, which happened to be perfectly still (stillness is comparable to death). He realizes that he will never be able to live forever, yet, the urn will. Because of this knowledge, Keats is obviously obsessed with having the ability to become immortal.
Keats is adamant about not confronting the ultimate fate that he is to pass away very soon. The urn, being a materialistic item, obtains no such realization. It is for this fact that Keats is a tad bit jealous of the urn's privilege to live for eternity as he will not. He points out that like art, nature can never stand still. Thus, the Grecian Urn's beauty will live on as the world will continue to change.
As the poem stretches on, the tone becomes sadder. Keats eventually admits that his love will never blossom like nature allows flowers to do. He points out that unlike true love, art is not real. It is for this very reason that this poem is sometimes misunderstood. Keats could take something so horrifically wrong and turn it into the most beautiful flower¾a complete genius.