Suddenly there flashed across his mind what he had said in Basil Hallward’s studio the day the picture had been finished. Yes, he remembered it perfectly. He had uttered a mad wish that he himself might remain young, and the portrait grow old; that his own beauty might be untarnished, and the face on the canvas bear the burden of his passions and his sins; that the painted image might be seared with the lines of suffering and thought, and that he might keep all the delicate bloom and loveliness of his then just conscious boyhood. Surely his wish had not been fulfilled? Such things were impossible. It seemed monstrous even to think of them. And, yet, there was the picture before him, with the touch of cruelty in the mouth.

– Oscar Wilde

The Picture of Dorian Gray, Chapter 7. As a change in the portrait reflects his recent ugly behavior towards Sibyl, Dorian realizes that the painting is a window to his soul and will mirror who he really is inside, while outwardly he would preserve his youthful beauty. He recalls the "mad wish" he made the day Basil finished the picture, that he would remain young and the portrait grow old. His wish had been fulfilled!