After Paul, and Darry broke up because of Paul's fear of being found out by his parents, and his internalized homophobia, Darry was crushed.

The gang could all see it. The way he acted, and how frustrated he got when Paul, hell, even just Socs in general were brought up. He really loved Paul, so it hurt. The guys decided to take it into their own hands.

See,

Paul has a younger sibling who's in Ponyboy'e class. They're fifteen, so the gang thought to confront them, and try and work something out with them to get Paul, and Darry back together without Darry realizes their plan.

What could go wrong?

Oh boy...