What important thing does Ponyboy finally learn at the end of chapter 6?

Ponyboy learns that that Darry can be afraid..... that he can be afraid of losing his loved ones.

He was stroking my hair and I could hear the sobs racking him as he fought to keep back the tears. "Oh, Pony, I thought we'd lost you... like we did Mom and Dad..."

That was his silent fear then--of losing another person he loved. I remembered how close he and Dad had been, and I wondered how I could ever have thought him hard and unfeeling. I listened to his heart pounding through his T-shirt and knew everything was going to be okay now. I had taken the long way around, but I was finally home. To stay.