Did you notice his thumb seemed to be 'stuck' in Jack's eye just for a second as he was stroking Jack's face in the second tent scene? One of the many subtle things that make the scene so realistic, and Ennis so endearing.
I did notice this, and you're absolutely right. The look on his face is a physicalisation of the close proximity of pleasure and pain. He can hardly belive he's doing this, and at the same time he's so overwhelmed he can't decide which part of Jack he wants to touch first. The speed at which his hand passes over Jack's face and chest is like he's almost afraid to linger in any one place too long. Like his words, the emotions are fighting their way out of him. So when Jack rolls on top of him he can't put his arms around him, but can only leave them in a position of submission - "Please Jack, you've gotta take the reins on this one!".
I think everything about that little scene of them riding next to each other is meant to convey the idea that they're in harmony at this stage and things are at a relatively smooth moment in their relationship (the color of the sky, the smooth way the sheep are all moving together... and especially Ennis's smile and adoring look at Jack, all make the scene feel really peaceful to me).
Totally agree Amanda (did I get your name right??
). And i think the way that Jack is slumped on the horse is important too. I think in practically all the other scenes when Jack is on the horse he's performing for Ennis, trying to prove his horsemanship to impress him. But here he's completely relaxed - nothing left to prove.
Another bit i really like is Jack's little nods as Ennis tells him of his plans to marry Alma after they've come off the mountain. Breaks my heart every time. Such a look of desperate resignation on his face