Why has the narrator gone to visit Usher?

An unnamed protagonist (the Narrator) is summoned to the remote mansion of his boyhood friend, Roderick Usher.

Its proprietor, Roderick Usher, had been one of my boon companions in boyhood; but many years had elapsed since our last meeting. A letter, however, had lately reached me in a distant part of the country — a letter from him — which, in its wildly importunate nature, had admitted of no other than a personal reply.