How does Wordsworth describe nature in his poem "Lines Written in Early Spring"? Why does he feel sad?

As the speaker relaxes in a lovely grove, they imagine the feelings of the other, non-human creatures around them. In this “green bower,” there are countless flowers, including primrose and periwinkle. The speaker imputes agency to these plants:

. . . 'tis my faith that every flower
Enjoys the air it breathes.

Similarly, the birds that flit and hop all around, and the “budding twigs” all show “pleasure.” However, this enjoyment not only emanates from the flora and fauna themselves but is evidence of God’s intentions as he perceives them: he has a “heaven sent” belief in “Nature's holy plan.”
The speaker’s deep appreciation for nature extends to distress over its being despoiled by humans. Their enjoyment of the sylvan bower is marred by knowing that such locales are becoming harder to find. The effects of human greed and neglect are destroying the natural world to which the speaker is so devoted, which causes him grief. The speaker first states and then asks rhetorically,

And much it grieved my heart to think
What man has made of man . . .
Have I not reason to lament
What man has made of man?
https://www.poetryfoundation.org/poems/51001/lines-written-in-early-spring