This is one of the famous so-called Lucy poems by William Wordsworth, that capture the life of the child Lucy who died during her childhood and never reached adult years. Even though this is quite tragic, at the same time, to Wordsworth there is a fascination with this figure and the way that children in his mind represent a clear connection with nature that enables them to be more in tune with the natural world and all of its mystical wonder than with the world of humans. Lucy, by dying young, never lost this connection, whereas most humans lose it when they become adults.
This is why the sibilance in the opening line of the poem with the repetition of "s" seems to create a rather dream-like, other-worldly feel to it. Lucy, when she is contemplated by the speaker, is a figure who seems separate and remote from human experience as she is a figure who cannot experience the "touch of human years."
The final stanza reflects on her early death and the way that she has no sight nor hearing. However, what she has not lost is that connection to nature, as, although she is dead, she is part of the cycle of nature and "earth's diurnal course" as she is "With rocks, and stones, and trees." Interestingly, the poem doesn't mention the word death which perhaps indicates the speaker's feelings about Lucy and her connection with nature. She has not "died" in the traditional sense to him, because she is still very much part of the natural world.