why does the narrator is insane in the tell tale heart?

This is the whole quote in context;

“TRUE! – nervous – very, very dreadfully nervous I had been and am; but why will you say that I am mad? The disease had sharpened my senses – not destroyed – not dulled them. Above all was the sense of hearing acute. I heard all things in the heaven and in the earth. I heard many things in hell. How, then, am I mad? Hearken! and observe how healthily – how calmly I can tell you the whole story.”

“And have I not told you that what you mistake for madness is but over-acuteness of the sense? --now, I say, there came to my ears a low, dull, quick sound, such as a watch makes when enveloped in cotton. I knew that sound well, too. It was the beating of the old man's heart. It increased my fury, as the beating of a drum stimulates the soldier into courage.”