Why does Ponyboy run away from home?

Ponyboy runs away from home in The Outsiders because Darry hits him after he comes home late, and Ponyboy interprets his reaction as proof that Darry does not really love him.

The immediate cause is the fight at home. Ponyboy has fallen asleep in the vacant lot after talking with Johnny, so he gets back much later than he should. Darry, who is responsible for Ponyboy and Sodapop after their parents’ deaths, is furious because Ponyboy’s carelessness could get him hurt or get the brothers separated. Sodapop tries to calm things down, but Darry loses control and slaps Ponyboy. Darry regrets it right away and tries to apologize, but Ponyboy runs out before he can hear him.

Ponyboy’s deeper reason is his misunderstanding of Darry. He often thinks Darry is only hard, critical, and unfair, while Sodapop tries to explain that Darry is strict because he is scared for Ponyboy and wants him to have a better future. Earlier, Ponyboy admits that he pretends not to care about Darry: “I don't care about Darry. But I was still lying and I knew it. I lie to myself all the time. But I never believe me.” This admission shows that Ponyboy’s anger is mixed with hurt and fear rather than simple hatred.

After running out, Ponyboy finds Johnny and says he has decided to run away. Johnny agrees because his own home is abusive and neglectful, but their plan is not yet fixed. They decide to walk to the park before making a final decision. The situation turns into something much more serious when Bob and the Socs attack them there and Johnny kills Bob in self-defense. From that point on, Ponyboy and Johnny are fleeing because of Bob’s death, rather than only because Ponyboy fought with Darry.

Ponyboy’s running away grows out of the Curtis family’s central misunderstanding: Darry’s expresses his concern as anger, and Ponyboy’s need for love comes out as defiance. The novel later repairs that tension when Ponyboy finally sees Darry’s strictness as love and protection.