I think Ennis felt obligated to Jack to provide the meals as camp tender at the time, although I also think that Jack pissed and moaned about having to eat more beans, which was all that was left of the supplies after the bear incident.Could also mean that Ennis saw an opportunity to show off his shooting skills to Jack and provide dinner at the same time. He seemed proud of himself after scoring the elk by the way he snatched the gloves from the rock. Jack was definitly impressed but teased Ennis about getting caught with it. Maybe there's a tie in later on when Ennis tells jack that he killed a coyote after their roles where switched. Ennis had to hear the shotgun blast echoes through the mountain and knew Jack was shooting at something when Jack attempted to shoot the predator, hence Ennis bein tired of Jack's "dumbass missin".There's light moments between them in the film and IMO this is just one of them.