Why does Darry now hate Paul Holden?

Paul Holden is Darry's former friend and Soc member, the first to challenge Darry at the start of the rumble. Pony mentions that Paul had played halfback on Darry's football team and used to be close with his oldest brother. Unlike Darry, whose parents died in a tragic accident and who was forced to abandon his dreams of playing college football in order to raise Pony and Soda, Paul was fortunate and had the opportunity to enroll in college.
As the two boys stare each other down, Pony notices that Paul views Darry with hate, contempt, and pity. Darry hates and envies Paul because he was given certain advantages and opportunities in life. While Paul continued to play football and earn a college education, Darry was forced to work two jobs and struggles to make ends meet. Darry also hates the fact that he is representing the Greasers, which Pony understands is a source of shame. Overall, Darry is jealous of Paul's fortunate life and resents the fact that Paul was given so many opportunities while he was forced to work to support himself and his family and abandon his dreams.

At the rumble between the Socs and the greasers, Darry steps forward to start the fighting, saying "I'll take on anyone." When Paul Holden responds to Darry's challenge, Ponyboy hears Soda react with surprise and perhaps dismay, and Ponyboy recognizes Paul as Darry's old friend from his football days. According to Ponyboy, they hate each other now, despite sharing history and good times in the past: "[t]hey hate each other because one has to work for a living and the other comes from the West Side."
Ponyboy blames their hatred on the differences in their positions in society, which have nothing to do with them as people. In the time since Darry and Paul were friends, a lot has happened to Darry; he lost his parents and took on the responsibility of his younger brothers, and so much of his potential to do more with his life was lost. Paul, on the other hand, seems to have enjoyed the privilege of his position and attended college, something that may have interested Darry once. Darry's hatred of Paul may come from a feeling of envy and resentment that Paul might not even appreciate everything that he has.