That's a very working-class attitude and it doesn't have anything to do with being in love with Jack.
It is. And yet...
Can I play the movie-vs-story game here, too, since I've been playing it all over the place?
I think this is another scene where the events are similar in both places, but where the dynamics are different.
In the story, the scene is the first indication that paradise is about to end:
Even when the numbers were right Ennis knew the sheep were mixed. In a disquieting way everything seemed mixed. And in the movie... well, Jack's bitching about Aguirre, and Ennis is being responsible, and aren't they just sooooooo cute together when they're like that?
And then when they're done sorting out the sheep, there's nothing disquieting or mixed about the emotions on Ennis's face. He teases Jack about the harmonica, he gives that lovely smile of his, and the sun sets over a mountain landscape that is even more beautiful than the two men. (Hey, indulge me in my scenery-swooning for a bit. I've been really restrained about it lately.
)
So even though, yeah, it's a pretty typical working-class kind of attitude, there's something about the whole dynamic of the sheep-sorting that seems... comfortable. Or domestic. Or idyllic, despite mixed-up sheep and f***ing Aguirre (or not). It's one of those artless, charmed happiness moments for me, at least.
(Though maybe it's just because I am in love with that landscape.)