What is the central meaning of Shelley's "Stanzas Written in Dejection"?

The central meaning of "Stanzas Written in Dejection, near Naples" by Percy Bysshe Shelley seems to be a contrast between his own "despair" (ln. 28) and the relative comfort he finds in nature. To find solace in nature is typical of Romantic poetry, as is the expression of heightened emotions. Both of these elements figure strongly in Shelley's poem.

The poem begins with a detailed description of a pleasant natural scene. Shelley describes a beautiful day, probably on the Gulf of Naples, as the poet references "waves" (3), and "the winds, the birds, the ocean floods" (8). While Shelley's imagery indicates that the scene is aesthetically pleasant, he also imbues the natural setting with his own emotions when he says "The City's voice itself, is soft like Solitude's" (9). This solitude that he feels in the surrounding environment is referenced in the title ("Dejection") and later in the poem ("despair" in line 28). In the second stanza, Shelley continues to use sensory detail to describe the nature scene, but midway through the stanza, he explicitly places himself in the scene by saying, "I sit upon the sands alone --" (14). He does,  however, hear in the sounds of the ocean, something that "share[s] in [his] emotion" (18).

From this point, Shelley transitions to a more detailed explanation of his emotional state. He feels as though he has nothing, according to lines 19-24, and judges that "Others [he  sees]" have more than he (25). He observes those other people as "Smiling" and experiencing "pleasure" (26), while "To me that cup has been dealt in another measure" (27). Shelley contrasts his own feelings of emptiness with what he see as the fullness of others' lives. The fourth stanza opens with Shelley returning to some of the ideas of stanzas one and two, though, as he writes, "Yet now despair itself is mild,/ Even as the winds and waters are" (28-29). While he is dejected, his feelings are tempered by the nature around him. He feels that he could weep and even die here in this setting. Shelley closes the poem by saying that he may be lamented after his death, but he feels that he will not be missed that much, "for I am one/ Whom men love not," (41-42). People will "regret" (42) that he is gone, but the memory of nature, of the day that surrounds him, will only bring pleasure: "this day ... / Will linger, though enjoyed, like joy in memory yet" (43-45). Again, Shelley prioritizes the beauty of the natural setting and its ability to yield positive emotions. Those emotions range from "joy" at the end of the poem to the relative comfort referenced in stanza four. 

So the central meaning of the poem seems to hinge on the relationship between the beauty and comfort of nature (and its ability to confer pleasant, long-lasting memories) and the speaker's dejected emotional state. This emotional state appears to be allayed by the nature surrounding him, while the causes for his dejection are related to society (especially from comparing himself to other people).