I also love the way Jack is so cheery when he goes to get Ennis from where he's sulking up on the hill.But I think the reason is this: Jack is so happy with Ennis, I think he thinks there's no way Ennis will be able to live without him now and so expects them to see each other, if not straight away, but soon. So, even though Jack's obviously sad to leave their beloved mountain, he thinks it's not the end of him and Ennis. Ennis, obviously thinks otherwise. He connects Jack with Brokeback Mountain, not real life, and so now he's leaving the mountain, he'll be leaving Jack. That's just the way it is in Ennis's mind. That's why when he first sees Jack taking the tent down he's so mad, not about the money, but because he's so happy and contented with Jack, that he's going to have to leave that behind, and go back to reality. It's heartbreaking, because he takes it out on Jack in a number of ways. Firstly, rejecting Jack's offer of money, then storming off and not speaking to him, and then turning the play wrestling into a fight....that really does show how Ennis works. He's so upset about leaving Jack, but he's constantly fighting these inner demons about how feels that he then turns into an all out fight. He almost gives in to Jack after Jack hits him and Jack goes to comfort him, but there's no way Ennis is going to let Jack hit him and not hit him back...he has to show he's a man.
So much to see and analyze in that scene.....it leads on to Ennis breaking down in the alley, and it's all connected. Ennis loves Jack, but just can't say it.