"I am jealous of everything whose beauty does not die. I am jealous of the portrait you have painted of me. Why should it keep what I must lose? Every moment that passes takes something from me and gives something to it. Oh, if it were only the other way! If the picture could change, and I could be always what I am now! Why did you paint it? It will mock me some day – mock me horribly!" The hot tears welled into his eyes; he tore his hand away and, flinging himself on the divan, he buried his face in the cushions, as though he was praying.

– Oscar Wilde

The Picture of Dorian Gray, Chapter 2. The portrait of Dorian is a constant reminder to him of the fleeting nature of his youth and good looks and the inevitability of growing old. So Dorian is upset every time he looks at the picture because it is a reminder that this will happen. His joy at first seeing the painting is now replaced by anger and jealousy. This shows his ever-growing vanity. When Dorian says that the portrait will mock him horribly some day, this is foreshadowing his ultimate downfall and death.