Darry and Pony do not enjoy the same easy relationship that Pony has with Soda. Where Soda is teasingly affectionate and treats Pony as a friend, Darry is more distant and standoffish. Pony judges him harshly for most of the novel but fails to see the difficult predicament that Darry has been left to navigate. As the oldest of the brothers, Darry is trying to act as parent to Pony and help ensure that he not only survives but is successful. He sees the potential in his younger brother and wants to do everything possible to help him become more than another Greaser.
Because he isn't actually a parent, this creates a lot of tension between the two. Pony actually runs away on the fateful night that he and Johnny flee town in part because Darry hits him.
However, after arriving at the hospital following the church fire, Pony suddenly sees his older brother with more maturity, undoubtedly due to all he's experienced with Bob, Johnny, and the fire. As Pony realizes that Darry is silently crying, he also recalls that his big brother didn't even cry at their parents' funeral.
This changes their relationship as he realizes that Darry is not his enemy. Instead, he recognizes that Darry sees what Pony can be and pushes him to excel in life:
Darry did care about me, maybe as much as he cared about Soda, and because he cared he was trying too hard to make something of me. When he yelled "Pony, where have you been all this time?" he meant "Pony, you've scared me to death. Please be careful, because I couldn't stand it if anything happened to you."
Pony apologizes, and this is the catalyst for a transformation in their relationship.