In Chapter 2 of Oscar Wilde's The Picture of Dorian Gray we find Lord Henry Wooton, Basil Hallward, and Dorian Gray at the precise moment when Basil has completed the portrait of Dorian.
We know that, from the beginning, Basil warns how he has given "too much of himself" into the picture, and how his portrait basically possesses Basil's deep and nearly-obsessive fascination for Dorian.
Right as they admire the picture, Lord Henry Wooton begins his sermon of hedonism to Dorian. In it, he conveys a form of magical spell that makes Dorian incredibly sensitive about beauty, youth, and other shallow things that, to Henry, are essential for life.
[....]..youth is the one thing worth having.... you don't feel it now. Some day, when you are old and wrinkled and ugly, when thought has seared your forehead with its lines, and passion branded your lips with its hideous fires, you will feel it, you will feel it terribly. ..... Beauty is a form of Genius—is higher, indeed, than Genius, as it needs no explanation. It is of the great facts of the world, like sunlight, or springtime, or the reflection in dark waters of that silver shell we call the moon. It cannot be questioned. ...... You smile? Ah! when you have lost it you won't smile
The relationship between Dorian and his picture occurs after this has been said. Dorian starts to panic about his own physical fate, pointing at how the picture will always be young and fresh while he will fade away with time. Suddenly, Dorian becomes jealous of the picture and wishes with extreme passion that it is the picture, and not he, who would grown old and decrepit with time.
“How sad it is!” murmured Dorian Gray, with his eyes still fixed upon his own portrait. “How sad it is! I shall grow old, and horrible, and dreadful. But this picture will remain always young. It will never be older than this particular day of June…If it were only the other way! If it were I who was to be always young, and the picture that was to grow old! For that—for that—I would give everything! Yes, there is nothing in the whole world I would not give! I would give my soul for that!”
Here is the moment when Dorian basically summons his soul in order to make it exist within the picture. This also means that, from now own, all of the sins of the flesh that Dorian will explore will not manifest themselves in his soulless body, but on the picture, where his soul is now trapped.
This is the reason when, in the end, a very young looking Dorian Gray has outlived people of his same age who are either dead or old, as nature dictates.When his sins saturate him and he faces his picture for the last time, he sees every disgusting sin he ever commits manifested in his pathetic picture. When he stabs the picture, Dorian Gray frees his soul from it and the picture restores itself to its original beauty. Meanwhile, Dorian Gray's body drops dead and turns into the rotten, awful and decomposed carcass of an old man whose dirty soul has taken over the body again.