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.