O.K. I'm gonna excplain ONE MORE TIME. Ennis arrives in camp to find Jack CHEERFULLY striking the--set (tent). Jack doesn't seem to have a care in the world (his feelings are hurt when Ennis refuses his loan of money). That's because Jack doesn't realize that Ennis's plans for the rest of his life have not changed a bit. he may not feel he has a lock on Ennis, but is far from feeling that the end of the summmer on the mouintain is the end of theire being together (for some years).
Ennis, on the other hand, immediaterly gets mad as hell, ostensibly at Aguirre's cutting them out of a month's pay, but actually at having the summer with Jack cut short---and Ennis knows, as Jasck does not, that Ennis's plans for gettingt married have not changed one bit. That's what engaged guys do; get married...so Ennis is miserable, and Jack remains cheerful --until Ennis socks him hard when Jack is trying to console him...at this point, Jack realizes that Ennis is ...saying goodbye, with a wallop-. There ain't gonna be n0 more Jack and Ennis.
I hope that's clear as mud. befroe the sock, Jack doesn't know exactly what's going to happen next, buyt he's confident it includes the two of them being together somehgow. Ennis is miserable because--through his own doing--he's not going to be seeing Jack anymore. The sock communicates this to Jack.