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.
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!