Ponyboy's decision to run away from home in the middle of a cold night was an impulsive reaction to the argument he had with Darry. Darry had never hit Ponyboy before, but the eldest of the Curtis Brothers couldn't restrain himself when he
... wheeled around and slapped me so hard that it knocked me against the door.
Darry instantly regretted his action, but his apologies didn't work on Pony, who was already well down the street and "pretended I couldn't hear." It was then that Pony decided to keep on running: He would run away from home since
It was plain to me that Darry didn't want me around. And I wouldn't stay if he did. He wasn't ever going to hit me again.
When he met up with Johnny, Pony explained that Darry had never acted this way while their parents were alive but
"Now he just can't stand me."
After he had relaxed for a while, Pony changed his mind, deciding that Darry "couldn't stop me from living in my own house." But before he could return, Pony was faced with another crisis that forced him to make good his threat to run away.