In "Ode on a Grecian Urn," why are "youth," "lovers," and musicians so lucky to exist upon the urn?

According to the speaker, the figures and everything else on the urn, including the trees, are lucky to be on the urn because they will never age or die or even experience winter. The tree will never lose its leaves or see the end of spring. The musicians will be forever playing their music and will never be tired. The lovers will forever be young and at the height of their love.
Gazing at the picture on the urn, the narrator at this point in the poem, midway through, is at the height of ecstasy, longing to be a figure on the vase. Note the number of exclamation points and the number of "happys" in this verse. The speaker thinks it must be the best thing in the world to be in that scene. He speaks to the image, saying to the tree:

Ah, happy, happy boughs!

He says the same to the " happy melodist" and the young lovers:

More happy love! more happy, happy love!

He is contrasting their state as a part of a work of art to the human state, in which time is not frozen but passes. How wonderful, he says, to be able to live forever in a moment of joy that will never end.