In "Lines Written in Early Spring," the speaker is reclining in a grove, listening to birdsong and enjoying the spring flowers, when he begins to feel rather sad: he cannot help but contrast the beauty and perfection he sees in Nature with mankind’s imperfections. In this way, the poet is describing an imperfect (unhappy) human contemplating an Eden-like scene:
To her fair works did Nature link
The human soul that through me ran;
And much it grieved my heart to think
What man has made of man.
So if Wordsworth is comparing and contrasting the works of Nature (beauty and perfection) with “What man has made of man,” then it stands to reason that he is describing the unnatural aspects of human industry: the wars, strife, and grief which lead to human suffering and unhappiness.
Wordsworth underlines this a little further in the fourth and fifth stanza, when he wonders if these beautiful works of nature are happy:
The birds around me hopped and played,
Their thoughts I cannot measure:—
But the least motion which they made
It seemed a thrill of pleasure.
He concludes that they are:
And I must think, do all I can,
That there was pleasure there.
Ultimately he decides that the happiness of nature must be a “holy plan.” Therefore, if Nature’s holy plan, its divine intention, is happiness, then the suffering, war, and strife created by Man--"what Man has made of Man"--is all the more regrettable:
If this belief from heaven be sent,
If such be Nature’s holy plan,
Have I not reason to lament
What man has made of man?
So our analysis shows that when Wordsworth uses the words, “What man has made of man,” he is referring to the works of Man, which are negative actions such as wars, producing sorrow and conflict, in contrast with the works of Nature--positive creations such as flowers and birdsong, which produce beauty and happiness.