What special quality of the birds and wildflowers does the speaker comment on?

In this poem, the speaker personifies both the wild flowers and the birds, watching them and comparing them favorably to "what man has made of man." While "sad thoughts" have come to the speaker while sitting pensively at rest in the grove, the special quality he identifies in the wild flowers and the birds is that they do not seem to experience such waves of melancholy while out in nature. On the contrary, the speaker observes the various flowers in the grove and comes to the conclusion that every one "enjoys the air it breathes." For the flowers, it seems to the speaker, life is intended to be pleasurable, and they appear to be enjoying their quiet existence.
Likewise, the speaker attributes the human quality of thought to the birds, although he concedes that he is not able to "measure" those thoughts as such. Still, watching the way they move—he chooses the active verbs "hopped" and "played," which we often associate with children and a carefree nature—the speaker believes that the birds, too, take "pleasure" in everything they do.
The observation of these behaviors in the wild flowers and birds of the grove fills the speaker with the conviction that nature was intended to engender pleasure in those who experienced it. As such, the contrast between their "thoughts" and his own feelings is a dispiriting one, and drives the speaker to "lament" what man has created of himself. Unable now to simply enjoy nature as the birds and flowers do, he suggests that man has developed himself into something different to what nature intended, and has become less happy as a consequence.