
Kinda going back to the original topic here...
I always felt that Jack's "I hope you know that if you don't ever know the rest" is basically his way of saying that he blames Ennis for making things the way they were... think about everything he said before that line...
Jack: Tell you what, we coulda had a good life together, fu**kn real good life, had us a place of our own, but you didn't want it, Ennis! So what we got now, is Brokeback Mountain! Everything’s built on that, that's all we got boy, fu**kn all! So I hope you know that if you don't ever know the rest!
Ennis: G*dammit...
Jack: You count the damn few times that we have been together in nearly twenty years, and you measure the short fu**kn leash that you keep me on and then you ask me about Mexico and tell me you'll kill me for wanting something I don't hardly never get!
If you look at it this way, Ennis has just told him that he might kill him if he was to learn all about Mexico etc. and Jack is basically coming back and saying "You have only yourself to blame for driving me to Mexico... I wanted to make a life with you, and you didn't want me except on your terms."
In truth, Ennis was entirely unfair to Jack pretty much from the beginning of their after-reunion relationship. "Sure, I'll spend time with you and all... but only when it's convenient for me. So don't ever contact me in between, don’t ever complain or get upset about not being able to see me, don't ever just show up out of the blue, and don’t ever say things that even smack of your feelings for me because they make me uncomfortable and I won’t respond or return the sentiment in anyway… Oh… and by the way… don’t ever see any other men. Women are fine… but no men!”
Hmm... frankly, in a situation like that I am surprised that Jack didn't give up years ago, and sadly, much as it hurts to think it... I believe that Jack had given up. The fact that he was talking about another man at his parents house suggests that, while he truly does love Ennis and would do anything to be with him, he had perhaps finally accepted that he and Ennis would never be together and had decided to try to grab a bit of happiness where he could. That defeated, look of pure pain that he has on his face as Ennis drives away... I think he looks that way because his heart has been broken once again, and he has decided that it is time to give up.
If so, one can hardly blame him. :\'(
(I am aware that I have pretty much just trashed Ennis into the ground... and I apologize if I have offended anyone. :-[ Truthfully I love Ennis and I can relate to his fear and anxiety... but at the same time, there are times during the movie when I want to grab him and shake him until his teeth rattle while yelling "You are such an IDIOT!!"

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