In Eclipse, Rosalie shares the story of her transformation with Bella. Her own fiancé raped her and left her die on the street. Carlisle found her. Because he couldn't save her life, he turned her into a vampire. Rosalie believes that death is a better choice, or a "happier ending," than immortality as a vampire: "If we had happy endings, we [vampires] would all be under gravestones now" (7.20). Rosalie's biggest dream in life was to have a child, which vampires can't do. Consequently, she maintains that Bella is making the wrong choice by giving up her humanity. Despite their different views on transformation, Bella and Rosalie establish a fragile friendship or, as Bella put it, "We weren't friends yet, but I was pretty sure she wouldn't always hate me so much" (7.126).
To find out more about Rosalie and her problematic relationship with Bella, check out her "Character Analyses" in Twilight and New Moon on Shmoop.