1. What is it about Jack and Ennis that makes them unable to resist each other? Does Brokeback Mountain have something to do with it? If so, why does their passion continue well after they leave Brokeback?
Seduced by the beauty of Heath and Jake, it's easy to reply to the first part of the question with "are you KIDDING?" but the boys were much less handsome in the story--Jack was a little man whose most notable features were curly hair, big thighs and big teeth, while Ennis was cave chested with a tiny torso and long bowed legs. What did they see in each other almost from the get-go? Well, clearly, both were living at least some version of their dream: being cowboys, so they admired that in each other. I would guess that they were drawn to the physical traits Annie describes, while Ennis, so deeply steeped in fear, admires Jack's courage as a bull-rider, one who has even won a (minor) prize. Jack for his part is drawn to Ennis' "uncommonly quick reflexes" and his "muscular and supple body made for the horse and for fighting." Each has something the other lacks: Jack has courage to put it on the line, Ennis is more the ideal cowboy type, with the height and strength Jack lacks; in the movie he is also all but silent like the classic cowboy.
Also, to put it starkly, they had major help from alcohol and isolation. Jack's previous summer, and, in the movie, his ready passing of the whisky bottle when Ennis admits his virginity, plus his sideways glance with "me neither," imply he has at least some m/m sexual experience, but whether he's Jake or the little buck-toothed guy, he's not going to start something that will get him beaten up without some help. The story emphasizes that both are very much aware of their isolation.
And quite clearly, a friendship develops. It's pretty obvious they are isolated personally, too: Ennis from the trauma of Earl, and Jack from his father's abuse. Did his father "know?" Was his father also gay and full of self-hatred, which he takes out on Jack? There's not enough information to guess, but either is possible. In any case, both would have been wary--Ennis of everyone, and Jack of anyone figuring out the truth. Forced together, they discover they like each other, "respectful of each other's opinions, each glad to have a companion where none was expected."
Brokeback Mountain had both everything and nothing to do with what happened between Ennis and Jack. The lengthy isolation gives them a whole month to build to the moment of discovery, and then weeks of opportunity to build on it, otherwise we may be sure nothing would ever have happened. In slightly different ways, both movie and story let them treat the idea of the Mountain as both a convenient shorthand for what they found up there--and as an excuse. When Ennis finds the Shirts, the moment is recorded with the haunting image of him seeking the scent of Jack which has long faded, leaving "only the memory of it, the imagined power of Brokeback Mountain of which nothing was left but what he held in his hands." Jack says "all we got is Brokeback," which says much the same thing: they've been riding along for 20 years on memories, but never got much further than the summer: only the Reunion was new, and a step forward. After that it was all just coasting. But Brokeback did have power: it set the scene, a harsh but genuine Eden, which allowed their love--as opposed to just their opportunistic sex--to begin. In that sense, Brokeback, or something like it, was essential.
The third part of the question, at least, has a simple answer: they expected nothing from their summer job, but time, isolation, friendship--and whisky--helped them to find love. Could they have loved each some other man? Maybe, probably. But they were each other's first, which is very powerful, and Ennis in particular would never have allowed any other man to get close. It was a "thing," and it was deadly. The passion continued because of the power of the summer, and its power in their memories. They expected nothing, and found love.
