I have always thought from the first time I watched BBM that the reason for Jack's causalness about leaving was because he fully believed that he and Ennis would simply continue with their relationship after they left the mountain. Look at how perplexed he is by Ennis' moodiness about having to leave earlier then expected.
He was obviously not happy about being separated from Ennis once he realized that was what was happening, but given that Ennis had said it was a "one shot thing" I think he didn't know how or even dare to ask Ennis if that still stood.(After all... not to long before that Ennis had socked him in the face for seemingly no reason! It's reasonable to assume that Jack thought he might do it again if he asked such a question.) I think that his mention of working there again next summer was his way of trying to find out what Ennis' intentions were about their relationship. Notice how crushed he looks when Ennis brings up his impending marriage, that was the confirmation that, as far as Ennis was concerned, their relationship was over. For me, the look on his face when he drove away, after looking at Ennis in the mirror, was heartbreaking... he just looks so hurt and resigned. Poor Jack! :'(
Just my take on things!
Hmmm...you're leading me to think through this scene in a way I had not before.
I think Jack's shirt thievery tells us that by that point he worried about never seeing Ennis again. Here's how I piece it together:
(1) When Jack tells Ennis that they are to come down from the mountain early, Jack is not crushed because he hopes / believes they will continue their friendship off the mountain. Ennis conflicted, sullen and ultimately silent because he can't reconcile his love for Jack with the future he faces.
(2) By the time Ennis is sitting alone in the meadow, Jack senses the sadness in Ennis of their soon parting. The look in Jack's eyes as he spies Ennis and heads over to him show recognition that this could mark an end.
(3) The punch Ennis throws Jack confirms Jack's fear earlier. So when Ennis and Jack change their bloody shirts, Jack swipes Ennis' as a reminder of him and their time together. Had Jack still assumed at this point they would keep in contact, what would the reason for the shirt have been? I suppose it *could* have been a reason to look Ennis up later (hey, I have your shirt) but I think it's more likely it was to remember Ennis.
(4) By the parting scene at the truck, Jack is painfully aware of how it's going to be. He asks hopefully about coming back next summer, but he knows how it had o be.
By that point, as much as my heart screams OFFER ENNIS A RIDE, DAMMIT! I guess I understand why it happened that way. To me it is the parting in that scene that turns the corner down the one-way street to tragic ddevastation