If we try to view the story of Dorian Gray from a purely realistic perspective—in other words, we ignore or at least downplay the magical transformation of the picture—the whole tragedy is rooted in Dorian's rejection of Sibyl. Her suicide then sets him on a downward course from which he never recovers. The changes in the portrait are a confirmation of what Dorian already knows about himself and the cruelty that has overtaken him.
So the answer to your question may lie in what causes his rejection of Sibyl. During the performance of Romeo and Juliet, her acting is apparently so bad, Dorian is humiliated. He's told his friends about her, about how special and wonderful he thinks she is, and she turns out an embarrassment to him. Yet, his reaction to her is partly, or mostly, due to the pompous and self-important way Lord Henry judges her, as he judges everything and everybody. Even Basil shares this arrogant attitude as they comment on the play.
Dorian at this point in the story is a...
(The entire section contains 2 answers and 827 words.)