When Keats looks at the painted figures and trees on the Grecian urn (in Ode on a Grecian Urn), he is not just happy that these apparitions will never fade, because they are art, and thus permanent; he is happy that they have never had to live, that they are not  Ah, happy, happy boughs!
In its Ode on a Grecian Urn, Keats looks at the painted objects with happiness because they never came to life and will be permanent, as art figures.