Ode on a Grecian Urn

This is a poem I have had to read several times for various classes throughout the years. I've heard several teachers and professors talk about it, but my reaction to it is always the same. It always evokes in me a sense of sadness, which is an odd reaction, I know. But I always feel sad for the figures on the vase, probably because of a small word choice on the part of Keats. He makes it sound joyous, "Ah, happy, happy boughs!", when speaking of the trees, but it's his use of the word 'can' instead of 'will' that makes me feel the opposite. The trees can't shed their leaves. Even if they want to. Not that the leaves never will fall, but that they can't. Not that the Lovers never will kiss, but that they can't.
It also seems interesting to me that a Romantic poet, who should have a greater understanding of Nature and its constantly changing ways, would choose to write about an unchanging scene.