why does johnny like it better when his father is hitting him?

When his father hits him, Johnny knows he has his attention. Those are the times that he knows his father is aware he's around.

Johnny says this after Ponyboy tells him about the fight with Darry. Pony is upset not only because Darry hit him, but because Darry is always nagging him. Johnny prefers it when his dad is hitting him because, as he says, it is these times when Johnny feels like his father knows that he is there. Hinton is letting us know that Johnny is often ignored by his parents, a theme that will come up again when Johnny realizes that his parents haven't asked about him after his disappearance.