Why does Winnie want a brother or sister?

From the text:

And another, firmer voice—her mother's—added, "Come in now, Winnie. Right away. You'll get heat stroke out there on a day like this. And your lunch is ready."

"See?" said Winnie to the toad. "That's just what I mean. It's like that every minute. If I had a sister or a brother, there'd be someone else for them to watch. But, as it is, there's only me. I'm tired of being looked at all the time. I want to be by myself for a change."