Final answer:
Usher's gloom is primarily caused by the chronic illness and impending death of his sister, which impacts his mental state deeply.
Step-by-step explanation:
The natural reason that Usher gives for his gloomy behavior in Edgar Allan Poe's "The Fall of the House of Usher" can be attributed to the chronic illness and impending death of his beloved sister. Usher expresses that this grave situation leaves him distraught and contributes significantly to his pervasive sense of dread and melancholy. It is clear from the text that the ill-fated fate of his sister deeply affects Usher's mental and emotional state, overshadowing all other aspects of life with a veil of gloom.