In stanza 3, the climactic crescendo of the five-stanza poem, the speaker uses the word "happy" six times. He has been looking at a Grecian urn that pictures young people heading out for a religious festival in springtime. On the urn, everyone is in a joyful mood. As the speaker finds himself becoming more and more identified with the painted figures in front of him, he bursts out in ecstasy. He thinks it must be wonderful to be on a work of art, frozen for all time at a moment of bliss.
The speaker expresses his feeling that art is joyful by speaking to what is pictured on the urn and saying, first,
Ah, happy, happy boughs! that cannot shed
Your leaves, nor ever bid the Spring adieu.
Here, he is speaking to the trees and expressing how wonderful it must be for them—how happy they must be—because it will always be eternal springtime for them. Then, looking at a musician playing a tune, the speaker thinks,
And, happy melodist, unwearied,
For ever piping songs for ever new.
He feels how joyful it must be to be forever playing songs without getting tired or the songs getting old and repetitive.
At his most ecstatic moment, he gazes at the young lovers on the urn and states,
More happy love! more happy, happy love!
For ever warm and still to be enjoy'd,
For ever panting, and for ever young;
All breathing human passion far above,
That leaves a heart high-sorrowful and cloy'd,
A burning forehead, and a parching tongue.
He says "happy" three times because he is feeling intensely how wonderful it must be to have love frozen at its best and highest point, forever "warm" or ardent with desire.
The speaker is contrasting the happiness of being in a timeless work of art with the sad fact that human beings must inevitably pass through time, growing older and experiencing "winter," pain, and unhappiness. He is expressing a common human desire to freeze time at a joyful moment so that it will never end.