"Beaches" (1988), a movie about a toxic friendship

I never understood why the film tries to convince us CC (Bette Midler) and Hilary (Barbara Hershey) have such a unique and loving bond when they spend most of the movie hating each other.

They barely know each other and become pen pals for life, then reunite and Hilary ends up sleeping with CC's crush. When they drift away, jealousy consumes Hilary because CC becomes a star and Hilary starts to throw insults left and reunite. They do make up but CC still uses Hilary to help her break up with a guy she quickly loses interest in.