Why does Sibyl stop acting and what is Dorian’s reaction in The Picture of Dorian Gray by Oscar Wilde?

In chapter seven of The Picture of Dorian Gray, Sybil delivers a poor performance on stage and later declares to Dorian that she will no longer act:

I hate the stage. I might mimic a passion that I do not feel, but I cannot mimic one that burns me like fire. 

For Sybil, falling in love with Dorian has completely reversed her feelings about acting. Before, acting represented the only "reality" in Sybil's life. She believed in the characters that she portrayed and felt she was part of their world. Since falling in love with Dorian, however, Sybil views life differently and can no longer take pleasure from being on the stage: 

To-night, for the first time in my life, I saw through the hollowness, the sham, the silliness of the empty pageant in which I had always played.

Dorian is horrified by Sybil's decision and declares she has "killed" his love. In Dorian's eyes, Sybil is no longer the creative genius he believed her to be and no longer capable of bringing to life the stories and characters he adores.

Ironically, Dorian calls Sybil "shallow and stupid," but that is really a reflection of Dorian's own character. That Dorian would fall out of love with Sybil over her decision to leave the stage demonstrates that he never truly loved her. His love was based purely on an ideal, not reality.