Roderick Usher is a boyhood friend who has written a letter to the story's narrator asking him to come for a visit to try to help him overcome his physical and mental illnesses. The tone of his letter gives further evidence that Usher is really suffering severely. The narrator has not seen his old friend in many years, but since Usher calls him "his only personal friend" he feels compelled to make this journey and to pay this visit. When he arrives at the gloomy House of Usher, he finds that his friend has not exaggerated his mental and physical ailments. He has changed greatly since the narrator last saw him.
The second paragraph of the story very clearly recounts why the narrator is visiting the house. In the first paragraph, the narrator describes himself riding "on horseback, through a singularly dreary tract of country" in bad weather and arriving at a point when he can finally see the house in question. The house seems foreboding. This raises the question in the reader's mind as to why the narrator would undertake an unpleasant journey to a house that does not appear to be an enjoyable destination.
In the second paragraph, the narrator gives the reason for his journey. When he was young, he had been good friends with Roderick Usher, the owner of the house. Although they had fallen out of touch for several years, the narrator had recently received a letter from his old friend asking him to visit. The letter gave the narrator a sense that his old friend was both struggling with some form of mental and physical illness and was desperate for the company of a friend. The narrator says:
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.
Thus the narrator agrees to visit his friend due to feelings of friendship and compassion.