It's next to impossible not to hear the happiness of the speaker in "Fern Hill." From the first line, through all of the first five stanzas, the speaker is praising his youth as a time of joy. And the poem's playful sounds and language only mirror that emotion, adding music and jauntiness to the poem's lines. If there's one thing the speaker hasn't lost, it's his ability to remember just how great being young felt.
The speaker in "Fern Hill" could've just said, "I'm happy and I know it," and then clapped his hands, but instead, he uses alliteration, rhyme and metaphor to figuratively express his happiness throughout the poem.
Although most of "Fern Hill" seems to be a poem about happiness, the last stanza flips the entire poem upside down and the speaker's tone changes drastically to show that his happiness was nothing more than naiveté.