Because to influence a person is to give him one’s own soul. He does not think his natural thoughts, or burn with his natural passions. His virtues are not real to him. His sins, if there are such things as sins, are borrowed. He becomes an echo of someone else’s music, an actor of a part that has not been written for him.
– Oscar Wilde
The Picture of Dorian Gray, Chapter 2. Lord Henry speaks to Dorian about how all influence is immoral. If a person is influenced you take away who they are, they become "an actor of a part that has not been written for him." That is precisely what happens to Dorian, who begins to act on Henry’s words and even speak like him, as Henry sets out on a mission to reshape and mold Dorian to act and think like him. Basil takes notice of this when he observes in Chapter 4: "It was so unlike Dorian to speak like that."