The man she loved was dead.
Whether Ozai killed him four years ago in the Agni Kai, last year when he began his reign with his predecessor's taunts and threats still fresh in his mind, or some time during the past few weeks, Mai didn't know. She only knew that the new Fire Lord was not the prince she fell in love, waited for, and fought for. Zuko never kept secrets from her; indeed, the more brooding a problem warranted, the more he confided in her until Mai could stop him from dwelling on it. Zuko never avoided her when he was sad or troubled but came immediately to her for solace and comfort. Zuko knew that peace with the Earth Kingdom was not only right and just but in the best interests of the Fire Nation. Zuko knew his father was an evil tyrant whose very presence was poisonous and wanted nothing more to do with the man. Mai knew Zuko; she understood him, loved him, respected him, and trusted him. She didn't recognize the Fire Lord who threw himself at the feet of a fallen tyrant, begging for advice on how to plunge the world back into war. Somehow, Ozai had transformed his son into someone Mai couldn't love or respect...
Or trust.
Learning of his secret meetings with his father had shattered her last hope. He promised to stop shutting her out, but he lied. He said he loved her, but how could he mean that when he had stopped confiding in her? He needed his father's support and approval more than he needed hers. The man who loved her was gone; she missed him, but she wouldn't find him here. Every encounter with the Fire Lord only served as a painful reminder of the lover she lost. It felt unfaithful to Zuko to continue living here with his replacement.
Well, there was only one solution for that.
Author's Note: Don't worry, Zuko; Jane Eyre left, too - Mai will be back.