Why does pony boy describe Dally what they look of triumph on his face as he crumples under the streetlight?

Ponyboy describes Dally's expression of triumph as he crumples under the streetlight to highlight the complexity of his emotions in that moment. Dally, who often embodies toughness and bravado, experiences a fleeting sense of victory despite his tragic fate. This juxtaposition emphasizes the harsh realities of their lives and the deep personal struggles Dally faced, revealing that even in defeat, there can be a semblance of pride or defiance. It underscores the theme of loss and the burdens carried by the characters in their turbulent environment.