When Darry slaps Pony, Pony feels lonely and like he has no family. Darry hurt Pony physically and mentally, made him run away. Darry has a huge impact on Pony, he is Pony's guardian and is responsible for Pony. When Pony ran away Darry felt so bad that when the found him at the hospital he cried. Pony realizes how much he needs Darry and how much Darry needs him.
When Johnny kills Bob it changes the entire story, because it causes the boys to run away. They were scared and Johnny did it because he was protecting himself and Pony, but doing that he killed someone else and changed Pony's life in so many ways. Pony would have died if Johnny wasn't there and that would have changed so many things for his brothers. Pony was the center of the story because he was the main character but this setting and this period really shaped who he is.
Johnny's death brought sadness to Pony and the rest of the gang but he was Pony's best friend and it really hurt Pony. Pony got to the point where he was telling himself "Johnny's not dead" and that he killed Bob and Johnny didn't. Pony was devastated and it didn't help that Dally died the same night. Johnny was like Pony's rock and he was the person Pony could trust and without Johnny he is changing into someone he's not.