Why does Two-Bit slap Pony across the side of his head?

Pony says that Darry is "Hard as rock and about as human... I bet he wishes he could stick me in a home somewhere, and he'd do it, too, if Soda'd let him." Two-bit says that Ponyboy is all wrong about how Darry feels, and Johnny says he's surprised that Ponyboy feels that way, since he thought they got along. He hits Pony on the head.