One thing Ennis can't stand anymore is getting off work two or three times a year, for a week at a time, without some compelling explanation.
Ennis doesn't like to talk anyway, and to explain to the foreman that he needs a week off for a "f***ing trip" (let the reader understand) in the mountains would be odd. Then to explain he needs another week off... then to compromise and say, okay; then november.... This is all very difficult for Ennis, who has fears that "everyone knows" about their relationship. And in fact, I imagine the ranch foreman would think it odd, and might even suspect something .... fishy.
Remember, in the past, Ennis would quit the jobs rather than have to talk, even in the most oblique terms, about his relationship with Jack. Of course, he could lie, but that's not exactly his style.
Little by little, starting with the "Thanksgiving from Hell" scene, Ennis is forced to actually think about and deal with his relationship with Jack. And he is basically incapable of doing so.... so he says, "I can't stand this anymore."