In this poem, the speaker is resting in nature in early spring, “In that sweet mood when pleasant thoughts/Bring sad thoughts to the mind.” And so he contemplates the beauty of the flowers and birds around him, personifying nature and imbuing it with peace and serenity and balance, and contrasts this with the imbalance of humankind – all the ills imposed upon man, by man.
The metaphorical quality of the poem can be difficult to spot, for it is not a one-off comparison but rather one that embodies the entirety of the piece itself. We can find it referenced in the line "To her fair works did nature link / The human soul that through me ran." The poet is comparing the “fair works” of nature to the human soul -- the natural world without to the natural world within. This further underscores the dismay felt by the speaker at the actions of man toward mankind, for if the two are comparable, how can it be that there is so much peace in nature, and yet so little peace among men?
This is further expressed by the human characteristics given to elements of nature – “every flower” that “enjoys the air it breathes,” the pleasure felt by the playing birds and the “budding twigs” in their effort “to catch the breezy air.” By using these human actions and feelings Wordsworth emphasizes the metaphor of man’s internal feelings being like nature’s instinctual actions, and by extension the speaker can only assume that each human being feels pleasure just from being alive. And therefore the speaker laments, again, man’s inability to let this pleasure lie.