Why was Coraline so, well, bitchy?

I enjoyed the movie and I overall consider it much better than the book, which, by Neil Gaiman standards, was actually kind of a slog to read IMO (even for a children's book). Which is pretty shocking seeing how short it was.

That said, Coraline in the book was very sympathetic and you constantly felt how lonely she was, and how her being "bratty" was mostly just her seeking the attention of her parents. The same applies to the film, but instead of appearing as a kind but ignored child, movie-Coraline is plainly hostile to pretty much everybody she meets from the get-go, often with little provocation. I guess this is further accentuated by Wyborn's presence, who is a movie-only character.

I understand that a lot of Coraline's frustrations and resentments in the book were internal, which would be hard to portray in a film, but I feel like the movie didn't show enough of her good side until it was too late, near the climax.