Ennis and Jack, indeed had different views at the premature ending of their summer. To Ennis, this was the ending of the most happiest time of his life, and the person who gave it him was someone he'll never see again. The friendship, the beginning of love, the sex, it was over. Ennis was angry and upset. He'll never see his Jack again. And maybe anger at Jack, didn't he understand what this means? And his reactions towards Jack was an ending, striking out at Jack, cutting it off, moving on to Alma.
I don't think Jack was that fatalistic, yes, their summer on Brokeback was over, but not them. Ennis made him see differently, that's why he never pushed Ennis, just ask him would he be back. Jack always took his ques from Ennis's actions.