Yay, you guys! I think that through our collective brilliance, we have hit upon some solutions to the mystery. Kea's idea that it symbolizes their relationship and the conflict to come especially makes sense to me.
They get assigned the jobs they do because it's practical; Jack has done it before. But the story could as easily have made Ennis the one with experience and the sensible choice as sheep herder. BUT NO -- Jack is the more experienced of the two. It's Ennis' first time. Hmm ...
At first, when they're apart, we see them gazing at each other from afar (Jack looking down at Ennis at night, Ennis looking up at Jack during the day). As Kea suggests, this represents the way they remain separated, yet thinking about and longing for each other, throughout their lives.
Remember Jacks words..." we should both be in this camp ".......
but in a way they never could be in the same "camp" their lives , Ennis fears and their roles kept them in separate "camps"....
Jack spent the rest of his life waiting.....it started up on that mountain.....it showed the contrast in what he had with what he wanted...
As Kea and Ray point out, what Jack really wants is for BOTH of them to sleep at the campsite, so they can be together. (Not to get into a whole nother debate about when Jack decides to make his move, but if you ask me he's thinking about it right then.) Jack doesn't want to let rules keep them apart. His "we should both be in this camp" is like "you know, it could be like this always ..."
Ennis, stickler for rules, doesn't even consider this option. He wants to do something nice for Jack, but also he doesn't mind being uncomfortable (sticking with Alma, giving up happiness) in order to follow instructions. Jack's not expecting Ennis to suggest this, he looks kind of startled; he's hoping Ennis will agree that they should both stay in camp. But now that Ennis HAS offered, what can Jack do but go along with it? But the switch means they'll be apart from each other, thanks to Ennis.
Meanwhile, aside from your objections, Jeff, I think putting Jack in the role of housekeeper is a way symbolically to set up the tension and conflict between Jack and Alma. The two shirt-washing scenes, as others have noted, underscore this.
When Alma is washing the shirt, Ennis comes in to the kitchen and gives her a perfunctory kiss. When Jack is washing the shirt, he's alone and looks vulnerable, and Ennis is far away. And as others have observed on other threads, that scene, occuring as it does immediately after Ennis discovers the slaughtered lamb, foreshadows Jack's fate. And Ennis, who volunteered to sleep with the sheep in the first place not only to be nice to Jack but because he believed the sheep needed protecting (following rules for the sake of avoiding death), was in fact unable to protect them (Jack).
Does this scenario sound plausible?
Meanwhile, all this got me wondering, what's up with the canopener? Does that mean anything? Jack is confident he can use a canopener -- in other words, thinks he can "open up" Ennis. But turns out he actually can't. Is that too much of a stretch?