Here's why:
* Peter is jealous: Peter is already annoyed by Fudge, who is constantly getting attention and causing trouble. Mrs. Yarby fussing over Fudge only intensifies Peter's jealousy, making him feel like Fudge is getting preferential treatment.
* Peter feels Mrs. Yarby is unfair: Peter is used to being the center of attention in his own family. When Mrs. Yarby gives Fudge all the attention, Peter feels like she is being unfair.
* Peter is frustrated: Peter is trying to deal with Fudge's antics and feels like Mrs. Yarby is making the situation worse by coddling Fudge. He's frustrated that he can't get a moment of peace.
In short, Peter is annoyed, jealous, and frustrated by Mrs. Yarby's fussing over Fudge. He doesn't feel like it's fair or helpful.