It is good to see this kind of thread reactivated.
It has been ages since we actually discussed the movie.
I had always thought that Ennis was so depressed. and put out with Jack, because
he was deeply reflecting the reality of the situation. He wanted as much as Jack
did to have a life together. He was wondering if he could disappoint Alma,
go back on his ingrained reaction to the death he saw, and put it all together. In order to come to the conclusion that he so desperately craved. Thus when Jack tried jovially to bring him out of his funk. He reacted with anger. (I am sure in his mind, he could not see how under the circumstances, Jack could be so light hearted.) He had so much emotion bottled up inside, that he simply erupted. I think it even surprised him. Thus the reason that he felt the need to apologize four years later for his actions. Even if it was not outright. He obviously had been sorry for it, for the entire time they were separated. "He more or less said it." He obviously figured out, too late by aplenty, "that I should have never let you out of my sights."