Why did dally hit ponyboy across the back?

Dally hit Ponyboy across the back to snap him out of his daze after the traumatic experience of seeing Johnny die. It was Dally's way of showing tough love, trying to get Ponyboy to focus and not let his emotions overwhelm him. Dally, who often used aggression to express his feelings, believed that this physical jolt would help Ponyboy regain his composure.