Hi Amanda! What a good way to describe it, as Ennis' way of emotionally detaching.
However, as a longtime Ennis apologist (I swear, some days it's a full-time job!) I must also note that Jack doesn't do the greatest job of saying goodbye, either. Ennis comes back to find Jack cheerfully taking down the tent (symbol of their most intimate moments!) and explaining Aguirre's instructions with no apparent regret. Ennis, on the other hand, is devastated, and Jack's breezy demeanor makes it worse. He snaps at Jack's offer of a loan not because he's insulted that Jack thinks he needs it, but because he's upset that Jack doesn't seem to share his frustration. And Jack's lassooing thing also seems too lighthearted and flippant to him. Even if Ennis were able to say a heartfelt, serious goodbye, it would be pretty hard to do with Jack appearing so nonchallant. Hard for anyone, let alone Ennis.
Also, and maybe I'm just a romantic (at least when it comes to this movie), but I have always thought part of the reason for their tussle is that, at some subconscious level, it is a way to have one last bit of physical contact when the sexual kind seems out of the question. Some guys in a long-ago thread mentioned thinking the fight looked fakey. I think it looks fakey not because the filmmakers didn't know how to stage a realistic fight, but because it IS kind of fakey. When they roll down the hill together, it seems to me like a substitue for a roll in the hay.
But then Ennis winds up injured, and he reacts in his typical way, by lashing out.