Why did ponyboy try to assume responsibility for the stabbing death of rob?

Ponyboy tried to assume responsibility for the stabbing death of Bob because he felt a deep sense of guilt and wanted to protect Johnny, who had actually delivered the fatal blow in self-defense. He believed that taking the blame would shield Johnny from the consequences of their actions, as Johnny was already in a vulnerable state. Additionally, Ponyboy struggled with the weight of the situation and felt that doing so might somehow alleviate his own feelings of helplessness and confusion.