The boys and girls in the narrator's family can't be seen talking to each other outside of the house – you know, because of the whole cooties thing. They "live in separate worlds" (3.1).
So the narrator is stuck with her little sister Nenny, who's too young to be her friend.
Someday, the narrator says, she'll have a best friend who will be cool and fun and understand her jokes.
Until then, she feels like "a balloon tied to an anchor" (3.4).