Here's why:
* He insists on his sanity, yet his actions and language suggest otherwise. He claims to be "very, very dreadfully nervous," and his obsession with the old man's eye drives his actions. This contradiction raises doubt about his mental state.
* His language is highly emotional and exaggerated. He uses phrases like "a low, dull, quick sound" to describe the beating heart, which suggests he's hearing things and experiencing hallucinations.
* He's overly concerned with minute details and seemingly trivial matters. His meticulous planning of the murder and his focus on the sound of the heart beating are indicative of a mind consumed by obsessive thoughts.
Overall, the narrator's initial portrayal in "The Tell-Tale Heart" creates an unsettling atmosphere and casts doubt on his ability to accurately perceive and report events. He is a character who seems to be struggling with a deep psychological turmoil, and his unreliable narration sets the stage for a chilling and suspenseful story.