"Fern Hill" is six stanzas of praising and then lamenting days the speaker spent at Fern Hill as a youth. And this speaker is stoked about running through the countryside. Throughout the poem, he talks about how happy he was as a youngster and how oblivious he was that youth was passing.
But at the end of the poem, the tone shifts dramatically from joy to lamentation. It's almost like singing, "If you're happy and you know it, think again!" What was a carefree bliss for the speaker turns out to be a fleeting joy that he ever can't recapture. What a bummer.