Explain the following lines: "Have I not reason to lament what man has made of man"?

Literally, what Wordsworth is saying here is something like this:
Do I not have good reason to be upset about what people have turned themselves, and each other, into?
If we look more closely at the poem, we can get an understanding of why Wordsworth believes the state of affairs is so lamentable. The speaker is an appreciator of nature, who is described in the poem as having linked "the human soul" to her "fair works"—that is, there was always supposed to be a strong connection between nature and the soul. People are supposed to be able to find solace and peace in nature, which should prove to them the existence of God.
If this is the case, however, how can it be possible that people are the way that they are? When he witnesses the beauty of nature as he does in this poem, the speaker is all the more confused as to how such beauty could exist in the same world as all men's evil. He does not specifically protest about any one thing that "man" has collectively done, but it is clear enough that he is unimpressed with the behavior of people in general and feels it is not a good reflection of the "soul" which was supposed to echo the beauty of nature.

This line appears twice in Wordsworth's poem "Lines Written in Early Spring." In this poem, Wordsworth is out in nature, enjoying the primrose, the periwinkle, the birds, and the spring breezes. All is beautiful and peaceful, and it seems to him that this lovely scene is sent by heaven.
Wordsworth contrasts the beauties of nature, which he describes as part of God's "holy plan," with the barbaric ways that humans treat other humans in civilization. He "laments" or cries out in sorrow, over what "man has made of man."
The poet-narrator doesn't specify what he laments. He doesn't need to, because he is assured that readers can conjure up their own mental pictures of the cruelties of war, poverty, crime and other problems that exist in manmade societies. That he laments or is sorry over these problems implies that society doesn't have to be this way. We could construct a gentler, more peaceful, more equalitarian society, where people would experience the same holy peace and delight that the poet experiences in nature.