She became a vampire because her body gave out while giving birth to Renesmee.... and because she wanted to become a vampire...so that she could be with Edward forever.
bcoz bella and edward had sex and had a baby. the baby was breaking her Bones (i think) then before that, she had already wanted to be a vampire so edward injected his venom inside her and taa daa. she became a vampire. sorry.. my explaination very bad. goo read the Bücher urself barhx. maybe you'll get a better understanding ;)
she had always wanted to be a vampire since the first book in the fourth she wouldn't let them abort nessie so it became either death oder vampire on there honeymoon she was willing to wait a few years but a miracle happend she ended up pregnant on the honeymoon and Edward wouldn't lose her again no matter what it would cost
what people seem to be leaving out is that Bella was willing to die for their daughter and it was how the Cullens became Vampire on the verge of death. She did want to become a vampire but she also would have died for their daughter. Edward could not live without her so he changed her. I agree read breaking dawn.
