The fight on the last day on Brokeback is one of my favourite scenes from the movie, there's so many layers to it, it says so much about the characters without either of them speaking barely a word.
You've got Ennis, like a kid who doesn't want to go home after a day out, totally gutted at the thought of having to leave; Jack's gutted too, but his main concern is for Ennis, so he tries to get him to talk, by throwing the rope over him, but Ennis walks away, so Jack turns it into a play fight,thinking Ennis might lighten up a little, maybe hoping it'll lead to one last chance to make love before they go. It works at first, and they're both rolling around laughing, then I think it hits Ennis that this is the last time they'll get to be like this and it's like his pain and frustration take over and all of a sudden he's taking the fight seriously. When Jack hits him, and he feels the physical pain to go with the emotional pain, it's as if, even though Jack is clearly distraught that he's hurt him, Ennis doesn't see Jack as Jack the man he's come to love, he just sees him as the reason for the desperation he's feeling. It's another of those moments that come full circle later on, Ennis's punch in that scene is him saying "It's because of you, Jack, that I'm like this..." in their last scene together
It's one of the things I love so much about this movie, the way all the emotions you feel are multiplied, when you see scenes like this and you emphasise equally with both characters. My heart breaks when I see Jack lying on the grass, holding his head, after Ennis punches him, but I can't blame Ennis for it i, I can totally understand why he did it. And Ennis paid the price by spending the next four years wondering if it really was because Jack was sore from that punch (and all it meant), that he thought he'd never see him again