Nostalgia
A lot of Wordsworth's poetry is nostalgic about something; usually it's childhood, but in this poem it's a certain relationship to nature. The poem complains that people aren't moved by nature, and the speaker clearly yearns for a time when they were moved by nature. When he exclaims, "We have given our hearts away, a sordid boon!" you can hear him recalling the time when we still had our hearts, when we still saw something in nature that was "ours."
Later in the poem, one feels as though the speaker were nostalgic for a pre-modern world of paganism. He says the modern world makes him "forlorn," and he would "rather be / a Pagan suckled in a creed outworn" (9-10). He would rather go back in time or experience the religion of a bygone era because at least then he would feel "less forlorn"; he could potentially see Triton and Proteus, which is a lot more interesting than what he sees now.