Because Darry is his legal guardian now and he sees how much Darry loves him when he is visited at the hospital. Darry worries for his younger brother and wants to make sure he is alright. At first Ponyboy thinks that Darry thinks of him as just a bother, but he comes to realize that Darry loves him just as much as his other brothers.
Ponyboy had been resentful of his brother since the death of their parents. He didn't understand what Darry had to do to keep them all together. When he's is the hospital, he wonders if he really asked for Darry and hopes he has the last thing he would want to do is hurt his older brother.
Ponyboy is concerned because he feels like he has just made up with darry! He feels like now he understands him and realizes why he did things so he (Ponyboy) doesn't want to hurt Darry anymore!
Ponyboy cares for Darry and vice versa. He did not want to appear as though he just cared for Soda, especially after they had gotten each other
