Why did Cupid leave Psyche?

Cupid left Psyche because she had broken her promise of not looking at him. Persuaded by her sisters that he might be a monster, while he was asleep, she took a knife and a lamp to peek at his face. If he was a monster, she would have stabbed him. However, recognizing him as a god, the god of love no less, she was entranced and rather shocked. Hence, oil from her lamp dripped onto him, and he awoke to find her looking at him when he did not want to reveal his identity yet. Therefore, he left, because he couldn't bear to live in a house in which there was no trust.

So desperate for him back, Psyche seeks out Venus for help. Eventually, Cupid and Psyche get together again, marry, she becomes immortal, and they live on Olympus together.