Rosalie explains to Bella that she wants to tell her the story of her transformation, to show her that she's making a mistake by giving up her human life:
Rosalie was born into a wealthy family during the Great Depression. Her beauty drew many suitors. She dreamed of a loving husband, a house, and, most importantly, a baby. Her dream seemed to come true when she met Royce King, son of a royal family. They got engaged and planned for a lavish wedding. Although deep down Rosalie knew that she didn't love Royce and he didn't love her, she didn't want to give up her dream.
One night Rosalie ran into her lovely fiancé and a group of male friends on the street. Incredibly drunk, they raped Rosalie and left her to die in the street. Carlisle found her. He couldn't save her life, so he decided to turn her into a vampire to be a companion for Edward. After the painful transformation, Rosalie murdered Royce and his friends to take revenge for what they'd done to her.
Rosalie admits to Bella that she's been jealous of her because Edward chose Bella over her. Bella learns that Edward has never been into any woman before, although Tanya, a gorgeous vampire from the Denali Clan, pursued him.
At the end of her story, Rosalie pleads with Bella to not throw her human life away:
"You have the choice that I didn't have, and you're choosing wrong!" (7.108)
Bella doesn't think so, but she keeps her mouth shut, happy to just have a fragile relationship with Rosalie.
The next morning in school, Bella becomes tired of her enforced imprisonment. When Jacob suddenly appears out of nowhere on his motorcycle, she jumps on it and they take off before Alice can react.