Cameron,
this is fresh in my mind because... well, not to self-promote, but I just wrote a little fanfic about that on toycoon's "Jack with Ennis" thread. Just the other day. And, thinking about that scene again, another thing struck me... it was still light, still so early, and Ennis was going up to the sheep. Earlier on, when he and Jack were just getting to know each other, he would stay with Jack as late as possible, till the moon notched past two... why the difference? Why is he leaving Jack so early, now?
I guess because of the way relationships progress. At the beginning, there's all this intensity... sense of urgency... but later, things get calmer. So maybe it felt okay to have an early night, because there would be the next night after that, and the next...
Except, of course, there wasn't, so even that comfort, if that's what it was, becomes sad because of the dramatic irony in it. They're acting as if they have all the time in the world... as if, in some sense, they are already in 'the sweet life'... but they don't, and they're not.
All of that gets a little bit away from your point - sorry - about Ennis doing his duty and going to the sheep instead of being with Jack. And yes, that is sad too. Although, I guess, logically, he would've had to be with the sheep at least some nights.
I think they just felt comfortable. A false sense of security and timelessness. Just my 'take'..