According to Aggie (American Girl)

As far as 11-year-old Aggie knows, everything is great. Friends, school, family…all are fine and fun and predictable. But one random day, Aggie's longtime friend Fiona starts to pull away, canceling plans and acting, well, kind of different. It's not like anything happened—or did it? There aren't any good answers, and Aggie is left wondering how she's supposed to act now that her friend seems to have moved on without her. It turns out, though, that a new friendship can turn up just when you need it most.