Ponyboy does not understand his oldest brother. He believes that Darry resents him and bullies him. Darry should have gone to college and now he is "stuck" caring for Ponyboy. Pony thinks that is why Darry is always so angry with him.
However, after the confrontation with the Socs, Sodapop tries to explain that Darry just worries about Ponyboy. Soda says that Darry loves Pony and gets so worried about his safety and his future that it comes out as anger, but isn't.
Pony doesn't buy it. He stops complaining in the moment, but in the narrative he expresses his doubt:
Darry isn't ever sorry for anything he does
It isn't until Ponyboy and Johnny are taken to the hospital that Ponyboy understands how Darry feels about him, and understands that Sodapop has been right all along. He admits that Darry loves him and that all his yelling comes from wanting to portect Ponyboy. This theme is repeated in the last chapter, when he says:
"If we [the brothers ]don't have each other, we don't have anything."
He thinks Soda is lying and that Darry doesn't care about him.