There isn't any real good reason for fighting except self-defense (Hinton 117).
When a Soc yells 'Greaser,' it typically means that they're going to jump the greaser. Being by himself. it must have been especially scary for Ponyboy because Socs are described as ruthless. One of the Socs even held a knife up to Pony and tried to give him a 'haircut.' The Greasers take pride in their hair so a haircut would be devastating. This dialogue shows how serious and terrifying the situation is.
With the abandoned church burning on fire, everything was chaotic. Not even the teachers were calm. Ponyboy and Johnny showed extreme bravery saving the trapped kids. From their dialogue, you can see how dangerous, urgent, and momentous the predicament was. Although Johnny died, he sacrificed himself for many other lives.
Before the rumble, Ponyboy is finding reasons for fighting; he learns others' motivations for fighting but he can't find a reason for himself. Pony is separating his individual identity from his gang's identity. He's conflicting within his mind why people fight. After hearing from Randy, Ponyboy also starts disliking the war between Socials and Greasers.