Who is the speaker in the poem "Lines Written in Early Spring" by William Wordsworth?

Since the sentiments and the themes suit Wordsworth's perspectives on nature and human interaction, we can say that the speaker is Wordsworth himself. But beyond the biographical context, a reader can also claim that the speaker of the poem is anyone who appreciates nature while critiquing society. 

He (Wordsworth/speaker) notes how the naturally blending sounds of nature ("grove") have put him in a sweet mood. He adds that this natural peace makes him consider "sad thoughts" and this has to do with humanity. 

In the second stanza, he says that Nature linked her (Nature's) good works to each human soul. In other words, as humans, we are spiritually and naturally connected to the beauty, peace, and joy that we see in the natural world. Again, at the end of the stanza, Wordsworth then wonders why there is so much suffering, hate, and disorder in the social world of humans. 

For Wordsworth, nature and all of its component parts seem to take pleasure in every moment. This is in stark contrast to the potential corruption that he sees in man (humanity). This pessimism stems from Wordsworth's disappointment with the outcome of the French Revolution. This poem also reflects Wordsworth's admiration for nature and contains his message that each individual should acknowledge this connection to nature and perhaps adapt his metaphoric notion that, like the primrose tufts, birds, and budding branches, each breath, motion, and reaction should be filled with pleasure and appreciation.