Why does Randy want to talk to Ponyboy?

The answer to your question lies in Chapter 7. However, to understand what Randy is referring to when he talks to Ponyboy, we will have to recall events from the preceding chapter.
In Chapter 6, Ponyboy and Johnny saved some schoolchildren from a burning church building. As a result, both boys were injured. In Chapter 7, we discover that Johnny is badly injured and is in critical condition. He is in severe shock, has third-degree burns, and his back is broken.
In contrast, Ponyboy only has a few burns and a bruise across his back. So, he gets released from the hospital in short order. A little while after he returns home, Randy (Marcia's Soc boyfriend) turns up to speak to Ponyboy. Randy appears anxious to talk to Ponyboy about his rescue of the schoolchildren; he wants to know why Ponyboy rescued them. Randy maintains that, if he had been in Ponyboy's place, he would have let the children burn to death.
For his part, Ponyboy asserts that he wanted to save the children, even if it doesn't make sense to Randy. Randy responds that he never expected a Greaser to do such a heroic thing. Then, he confesses to Ponyboy that he has no intention of attending the upcoming rumble. Randy is still in deep grief at the loss of his good friend, Bob, and he's come to the conclusion that all the fighting and killing has become meaningless to him.
Ponyboy assures Randy that he would have saved the children as well if he had been in Ponyboy's place. Ponyboy's vote of confidence comforts Randy, who now understands that Socs and Greasers are more alike than they dare to admit. At the end of the conversation, Ponyboy has a similar epiphany: "Socs were just guys after all. Things were rough all over, but it was better that way. That way you could tell the other guy was human too."
So, on the surface, Randy may have wanted to talk to Ponyboy about the reasons for Ponyboy's altruistic action. In reality, however, Randy is desperate to reclaim his own humanity in the midst of the violent life he has led. Randy has a deep need to recognize both the humanity in him (a Soc) and Ponyboy (a Greaser) in order to justify his decision to stay away from the upcoming rumble. This is why he wants to know why Ponyboy acted as he did.