I also think the confrontation was weighing on his mind. I'm not sure how soon this scene is supposed to be after that, though. And in his conversation with Cassie he does his "blame" others thing ( I hate the word blame but can't think of how else to describe it) "Looks like I got the message in any case". That reminds me of the "once burned" comment he gave Alma. This scene makes me wonder what would have happened if they'd had one more meeting. He apologizes to Cassie here and then she defines things for him- it not being "fun" but love and he just has this look like some piece to the puzzle clicks in place. I'm sure he knew that he loved Jack but was maybe able to explain it to himself as loving the time he spent with Jack as opposed to defining "this thing", if that makes any sense.
You are so right, LJN. The "blame" thing. It reminds me of what Shaun told Zach in the car..."what do you call that?" It's exactly what Ennis was doing...feeling powerless, unable to change his circumstance, so he blames anyone and anything convenient, including Jack..."It's because of you that I'm like this."
Sulking and blaming the whole world while eating pie in the station, Cassie's comment made him realize what he could have done for love, instead of just for fun. Both Cassie was to Ennis, what Shaun was to Zach, the emotional mentor that both men needed. If only Ennis did what Zach did, and take what he wants...which, I think, was why he sent the postcard that got returned. Imagine if Zach went to Shaun's house, and found the guy left town already. Or, Aaron at Christian's apartment, thinking the new guy moved in. Losing Jack, Ennis was as devastated as Aaron was. Then the old man going on about the new fella, the ranch neighbour from Texas...It made the hurt even worse, the betrayal, the loss.
Jackster pointed out the contrasting technique that really brought out the power of those last scenes. All of this built up to the moment of realization, of discovery, or finding the two shirts, knowing Jack was never sore after the punch, that Jack kept their memories, alive, unwashed. To lose love and find it again, like that song Tuesday 3 AM, but in reverse, crawling through hell, and then being lifted to heaven. Ennis could not help but change in that instant.
We were all changed, and that's why we are here. To celebrate the greatest movie ever made.