Could also be Jack had a good understanding of Ennis's make up; that pushing him was a sure way to have Ennis back off completely or lash out. Ennis did so on Brokeback (that punch); shot down Jack's dream pretty firmly when ranching up together was tentatively suggested; certainly when Jack suggests off-the-cuff that Ennis try someplace new like Texas he gets sneered at; and we see how Ennis breaks down at the end when Jack's done with pretending that things are fine the way they were. So, confronting Ennis Del Mar too strongly seems to be a sure way to get rough words or a fist thrown your way .... 
Why didn't he stick around or push at that cursed divorce scene? Maybe because the girl's were right there so having it out right away wouldn't sit well with Ennis? But really, at that moment, I think ('cause I'm a Jack fan, yeah, I know!) that maybe the hurt and humiliation was just too deep for him to do anything other than get the heck out of there as fast as he could, before he broke down or said something to end the relationship permenant.
I know, I'm making excuses .... 
but you're really set out the worms for me on this one, ww. :-
You know, Jack had set himself up with a comfortable life during those four years apart. He still risked a lot by taking off to be with Ennis a couple of times a year, and driving hundreds of miles for the privilege of having whatever piece of Ennis's life the man would share. Maybe that was all the fight Jack had in him???
oh, ahdunno!! 
circles, always circles ...
Oh, I think you're right Missis..the slightly raised eyes when Jack says "gonna do this again next summer" is Jack willing Ennis to say "Yes". That is as much as he dare do I guess. Telling Ennis he wants him to without saying it. If Ennis had said "I don't know what I'm gonna do yet" Jack would have maybe seen an opening, but Ennis doesn't, he says he's going to marry Alma. Whats the point of Jack trying to say something then ? He's just been punched remember, he knows Ennis is on the edge of his nerves and it's not the time to push too hard. I've said before I think that Jack hoped that if Ennis was given a year (till the next summer) he may have had time to think about things. If they parted on a row, things could be said that would stop that happening, and right now, trying to talk to Ennis is only going to end bad. I know that's assuming that Jack already knew he loved Ennis, but I've always thought that he did know. Even after FNIT Ennis had to be left on his own to think about things before SNIT. If Jack had said anything more in the morning than "see you for supper" it would have been disastrous. Well I think Jack thinks the same here. I think Jack would love to fight harder and he would if it was someone else...but Jack knows his man.
On the mountain after the reunion, it's the same thing. I think that he just
KNOWS that Ennis is not a man to be pushed. He's just glad to know that he is going to be seeing Ennis again and privately thinks that over time, maybe Ennis will change his mind. He's not going to risk a row or being all or nothing now, because he probably knows it's the nothing he would get. Jack's not a coward, he just knows his man.
After the divorce scene, I think Jack is hurt
SOOO BAAAD that he is going to burst into tears if he stays a minute longer. The girls are there, the way Ennis looks at the truck on the road, everything is pointing to the fact that he was going to get nowhere at that time. All the times before Jack has been able to convince himself that maybe Ennis' circumstances will change and they'll be together if he waits long enough. This time even he can see that it's not events but Ennis himself who is stopping his dream from happening. He is stunned at the realization. He is also humiliated. And I think he's allowed to be a bit angry don't you?
He Just had to get away. He's not going to cry in front of Ennis, and he knows the tears are there. He's not going to beg. He jumps back in the truck and has put miles between them before he calms down himself. He needs time to think about this. He doesn't want to talk to Ennis until he's thought it through himself, because he knows that as always one wrong word from him and who knows what would happen.
By the time he see's Ennis again he's decided that, risking taking off a couple of times a year and driving hundreds of miles for the privilege of sharing whatever part of Ennis that Ennis was prepared to give was going to have to be enough. But I think the optimist in him always hoped that one day Ennis would come round. It wasn't that Jack couldn't fight, he just knew that he'd lose if he did. He was always, waiting, waiting, waiting. He didn't have to tell Ennis any of this. Ennis wasn't stupid, he already
knew. He wouldn't mention it, but he knew alright. That's why at the end when Jack finally explodes and does tell Ennis some truths Ennis is the one to be stunned. As always only IMO.
