Why does the narrator go to the house?

Usher sent the narrator a letter requesting a visit.

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. The MS. gave evidence of nervous agitation. The writer spoke of acute bodily illness--of a mental disorder which oppressed him--and of an earnest desire to see me, as his best, and indeed his only personal friend, with a view of attempting, by the cheerfulness of my society, some alleviation of his malady.