Why did Carly play as sams girlfriend in transformers 3?

Carly played as Sam's girlfriend in "Transformers: Dark of the Moon" to provide a romantic subplot that added depth to Sam's character and his motivations throughout the film. Her character, portrayed by Rosie Huntington-Whiteley, also served to introduce new dynamics and conflicts within the story, as well as to contrast with the established characters and relationships from the previous films. Additionally, the change in romantic interest aimed to refresh the narrative and appeal to a broader audience.