Why do Sam’s friends think she was quiet and didn’t want to go to the party?

In Lauren Oliver’s young adult novel Before I Fall, Sam is living the same day over and over: February 12th. The first day, she wakes up as normal. She goes to school with her other popular friends, teases the kids that aren’t as cool as her, goes to a party, and fights with her boyfriend. On the ride home, the girls get into a car crash and die. But the next morning, Sam wakes up and realizes that it is once again February 12th. The day repeats: school, party, accident, and then death.
Sam’s not sure why she's reliving the same day but believes it is an opportunity to save both herself and her friends. She thinks that if she is able to prevent them from going to the party, there won’t be a car accident and they will all be okay. However, her plan doesn’t work out. While she convinces them not to go to the party and have a sleepover instead, there is still death: the girl they always bully, Juliet Sykes, has committed suicide.
Sam, of course, doesn’t explain to her friends that she’s reliving the same day over and over. Her friends think that Sam is just trying to avoid her boyfriend, Rob. They all knew the couple had plans to sleep together at the Valentine’s Day party, so they believe that Sam just isn’t ready and is trying to avoid him.