I don't want to be tough on Ennis (like I often am), but I am not sure how much he understood altogether about wanting someone.
Well, his reaction in FNIT when he just flipped Jack shows that he had pretty good idea how the things go, but was he really really aware of that situation in his mind before Jack took his hand to warm it up
? I am not sure... 
But, maybe you can convince me as you often do... 
I think Ennis was aware,or at least suspicious,that something was going to happen because he didn't want to enter into the tent in spite of the cold.This is IMO one of this "so Ennis" ambivalent situations we see sometimes in the movie; he wished that something happenned and,at the same time,he was afraid that this something could happen-surely also afraid of wishing it-.And,so,he suffered from the night cold without entering into the warmtn-climatological and human-of the tent,but at the same time,he didn't make anything to revive the bonfire;as a way to avoid and at the same time,provoke the situation he wished and feared;as I said formerly,"with or without you"...