In The Outsiders, why is Paul Holden's acceptance of Darry's challenge significant?

Before the rumble begins, a handsome, athletic Soc named Paul Holden steps out from the group to challenge Darry, who represents the Greasers. Ponyboy mentions that Paul Holden used to be teammates with Darry, and the two boys were inseparable as teenagers. He goes on to say that Paul looks at Darry with contempt, pity, and hatred as he is sizing him up. Ponyboy then mentions that he could tell Darry hated Paul and was filled with jealousy because Paul had achieved everything he could not because of his difficult situation. Pony also knows that Darry feels ashamed to be representing the Greasers.

The face-off between Darry and Paul is significant because Paul Holden represents everything that Darry could have been. Paul is a privileged, athletic young man who had the opportunity to go to college, which is something Darry never had a chance to do once his parents died. Darry envies his former friend for being given the opportunity to play sports and attend college. Their face-off also underscores the class distinction between the two groups and differentiates Darry from the rest of the Greasers.

What is ironic about Paul Holden accepting Darry's challenge first in The Outsiders?

The irony is that the boys are much the same and, besides socioeconomic status, share a common background. They were both great athletes in high school, and they "used to buddy it around all the time." They are both fairly intelligent and Pony mentions that Darry had earned an athletic scholarship to go to college, but there simply wasn't enough money to make it happen. But while Paul comes from a background privileged enough to send him to college, Darry has become the guardian of his younger brothers following their parents' death. He works jobs requiring physical labor to provide for their family, and college has never been an option for him. Two-Bit also realizes that in some ways, their gang has held Darry back from reaching his fullest potential: "You know, the only thing that keeps Darry from bein' a Soc is us."

When the fight is about to commence, it is Darry who emerges as leader of his group, offering to take on anyone. And it is Paul who rises to meet this challenge. This presents a face-off of young men who have so much in common and have come together to fight because of an ultimate conflict over their backgrounds.

Paul looks at Darry with contempt, and Darry returns his stare with hatred. Neither one has personally wronged the other, yet they plan to fight, anyway. The irony is that if Darry had been allowed different advantages, he and Paul would likely be living very similar lives at this age—not preparing to fight.