Why does the narrator visit the house of Usher?

The narrator went to visit Usher after receiving a letter asking him to do so.

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. It was the manner in which all this, and much more, was said - it was the apparent heart that went with his request - which allowed me no room for hesitation; and I accordingly obeyed forthwith what I still considered a very singular summons.