In this scene Ponyboy is slapped by his older brother and the eldest Curtis brother, Darry after a very heated argument. They are all shocked because Ponyboy had never been hit by a member of his family. Soda, the second eldest brother, is sitting on the couch watching this scene unfold.
In this scene Ponyboy is running away because his brother, Darry, slapped him. He always thought that Darry never loved him and that he only loved Soda, his other brother. When their parents died Darry started working and taking care of the family, he also assumed the habit of treating Pony as if he were glass and becoming defensive when he is worried for Pony.
I can't believe Darry just slapped me. My own brother, I knew he didn't love me
In this scene Ponyboy finds his best friend Johnny near the Park and tries to get Johnny to run away with him. Nevertheless Johnny convinces him to go back home and only talk to Soda instead.
Maybe we should go back, I mean Soda is there and I know he loves me. Who knows maybe both my brothers do.