Theme of "Ode on a Grecian Urn"

The poem "Ode on a Gracian Urn" by John Keats doesn't quite paint a picture of the meaning happy in your mind. Though the poem cleary states happiness many times it simply is not believable. The speaker is admiring an ancient Greek urn with several paintings on the outside. He proceeds to compare the pictures with his life. "Heard melodies are sweet, but those unheard are sweeter; therefore, ye soft pipes play on;" he debates whether it is better to hear the melodies the musicians are playing or if it is better to imagine it and see the painting itself(lines 11-12). Then during stanza three the tone changes when "that leaves a heart high-sorrowful and cloy'd" because the speaker realizes that this is not life but just a picture. The speaker continues to look at the different paintings on the urn when in the end states, "Beauty is truth, truth is beauty"(line 49). The speaker knows the urn is never changing for centuries to come it will always remain beautiful. In real life this cannot be true, things change, people change, and leaves on a tree change in real life.