Two themes that leap out in this poem are melancholy and beauty. See the quote: “I might/As happy be as earth is beautiful.”
The rich scene has grown fresh again and new
As Spring and to the touch is not more cool
Than it is warm to the gaze; and now I might
As happy be as earth is beautiful,
Were I some other or with earth could turn
In alternation ...
The poet isn't happy, however. Describing what appears to be a case of clinical depression, he wonders how he can feel so melancholy when the natural world is so bustling. Even though it is October, and winter is about to come, “the rich scene has grown fresh again and new.”
Still, the only happy thing that the poet can say about his feelings is that it's a good thing that his mood will pass with time.
“But if this be not happiness, -- who knows?
Some day I shall think this a happy day.”
In short, to be this poet is rather depressing, but then again, Edward Thomas' other frequent theme is war.