In Chapter 7, Dorian's narrative supercedes all others in the novel. From now on, it will be his story, not Lord Henry's. The novel becomes more dynamic because Dorian's character grows — changes — while Lord Henry's remains unchanged.
The change in Dorian's character in this chapter is dramatic. Dorian begins the chapter as a dedicated lover. Then, in a few short pages, he becomes a disgusted critic, a heartless deserter, briefly a contrite sinner, and then finally a lover rededicated to Sibyl — not because he loves the woman, but because he fears hurting himself and the portrait. Even though the chapter ends with Dorian intending to do "his duty" by being honorable and marrying Sibyl, his honor is false because it is based on selfishness. His "honorable intentions" are simply a continuation of his soul's degradation. The number and degree of changes that Dorian goes through in this chapter, most of them negative changes, hint at the turn his nature will take in the rest of the book.
Chapter 7 also introduces an element that will reoccur throughout the story: the changing of the portrait. By the end of the chapter, the reader understands that the portrait will symbolize the state of Dorian's soul and spirit. Wilde will use the portrait to help develop his characterization of Dorian for the rest of the book.
Dorian's special relationship with his portrait continues the Faust theme. His wish about the portrait suggests a pact with the devil. Dorian's desire to escape the "poisonous theories" of Lord Henry indicates that he sees his mentor as an evil, devil-like influence, but, like Faust, Dorian seems eager to benefit from the fruits of his pact, namely the eternal youth that the portrait offers him.
