Nature
As a Romantic poet, Leopardi is predictably concerned with the natural world and its beauty. Unlike other Romantic poets such as Wordsworth, Leopardi's depictions of nature are often colored by a feeling of melancholy and bitterness. For example, in "To Spring," Leopardi depicts this sprightly season as being a reminder of death and decay. This unique depiction of nature, although beautiful, is something that defines much of Leopardi's works.
Disillusionment
Leopardi himself had a difficult upbringing and adolescence. Under pressure from his parents, who had very high expectations, Leopardi suffered health conditions, including reduced eyesight, due to intense studying. In many of his poems, Leopardi depicts a feeling of nostalgia and loss, looking back to happier and healthier times. This is a key theme of his poem "Memories," in which a poet-figure thinks of his boyhood while looking out of a window.
Disappointment
In many ways, Leopardi is an embodiment of the suffering artist; he had a difficult life and found an escape through his creative pursuits. In his poem "To Sylvia," Leopardi symbolizes his disappointment and disillusionment with the world in his depiction of Sylvia's physical demise.