Here's why:
* Bella's attraction to both Edward and Jacob: She is deeply in love with Edward, a vampire, but also has a strong connection with Jacob, a werewolf. Both relationships are fraught with danger and complications.
* The danger of both worlds: Bella's presence puts both vampires and werewolves at risk, making her a target for both sides.
* The choice between two impossible options: Bella faces a difficult decision: choosing Edward means potentially endangering Jacob and the werewolf pack, while choosing Jacob puts Edward and his family in danger.
Although not a direct quote, the phrase "caught between a rock and a hard place" effectively captures the essence of Bella's predicament throughout the series.