As the reader learns early in the novel, Dorian Gray is devastated to hear from the cunning Lord Henry how the tragedy of life is that the soul grows young, but the body grows old. Dorian absorbs Lord Henry's words and realizes in complete horror that his fate will be quite similar. As a result, he makes the wish that seals his fate forever:
“How sad it is! I shall grow old, and horrible, and dreadful. But this picture will remain always young...If it were only the other way!.. For that—for that—I would give everything! ... I would give my soul for that!”
Although destiny somehow grants Dorian his wish of remaining young while his picture grows old, this grant does not come cheap; the picture not only shows age, but also the horrid looks of evil and sin. This is because both Dorian's age and soul are now trapped inside the picture while a soulless, haunted Dorian walks the streets of East London sampling the sins from the underbelly of a dark city.
As Dorian's persona becomes the vessel of pure sin, his portrait deforms into the face of a horrid creature that reeks of horror and havoc. Each time Dorian commits a sin, the picture shows it in its face. Yet, far from this serving as a lesson for him to change his ways, he turns even more sinful by admiring his flawless beauty and looking mockingly at the picture as if it were yet one more of his servants. This is all part of Dorian's excessive narcissism, as well as of the deep and negative influence that Lord Henry's canon has caused on Dorian.
Therefore, the way that the picture distorts, oozes, and transforms so horribly serves as ammunition to further feed Dorian's hedonistic and sinful behavior. It is precisely the credo of hedonism what Wilde intends to praise in this highly-aesthetic novel and it is through Lord Henry that he accomplishes this effectively.