Girl friendships are fierce and dangerous. The expression “I’ll be your best friend” is not exclusively a girl phrase, but girls use it a lot. They know how powerful the words are, how significant the offer is. Girls who become good friends feel a compulsion to define the friendship, to stamp it and name it, and they are inclined to rank a close friend as a best friend, with the result that they often have many best friends. They think about their friends on a daily basis and try to figure out where a particular friend fits that day in their cosmology of friendships. Is the girl her best friend today, or a provisional best friend, pending the resolution of a minor technicality, a small bit of friction encountered the day before? The girl may want to view a particular girl as her best friend, but she worries how her previous best friend will take it - as a betrayal or as a potential benefit, a bringing in of a new source of strength to the pair. Girls fall in love with each other and feel an intimacy for each other that is hard for them to describe or understand.
— Natalie Angier in her essay “Indirect Aggression”