I've seen that as Jack turning pensive and sad, compassionate, because he's realizing and taking in the depth of Ennis's previous loneliness. Jack's reaction was especially poignant because it contrasted with the film audience's reaction (at least over here) - people laughing at Ennis's statemen. Audiences initially saw it as a mere joke and nothing but a joke, but Jack's reaction more than anything showed them the reality behind Ennis's words.
Also, I've been wondering about Jack's "boot in the fire" - whether that was intentional (and if so, to what purpose?) or just happened that way, with Jake continuing with the scene and Ang Lee deciding to keep that particular take later on. In a way it makes it look especially realistic.
And finally, I've wondered about those Forest Service people that weren't to catch our sheep herders with no elk; - wouldn't the Forest Service have reacted and been suspicious about that very visible rack with lots of drying meat, in the middle of camp?
"Oh, sorry, but we came across this poor elk that had broken its leg so we had to help it out of its misery.... And no use wasting good food, is there? "