He growls right back, the posturing of an animal trying not to cede territory. He's frustrated and embarrassed and nervous and he barely even knows which of the knotted up buzzy emotions in his chest apply to those words. There's so many.

"B- Because I fucking- Because you- Because..." Why is he following John? All his reasons are embarrassing. "Because I want to know why you're better than me!"

And as soon as he yells that he turns into a sparrow and tries for the nearest window.