I've always thought that it was mostly lonliness that brought these two guys together ( along with circumstance). Neither had a strong tie to a family, nor lots of friends and associates. The jobs they went after were lonely, as was the job on BBM. Although Ennis had Alma waiting at home, he was willing to leave her to look for work which kept him away from her. And, although Jack had a mom and dad, they lived such a bleak, isolated life, he was willing to go on the rodeo circuit and take odd jobs to pay his entrance fees. So, I think both men were used to being solitary, making decisions for themselves about their own lives, and not asking help from anyone.
Keeping that in mind, then, this togetherness into which they were thrust in the BBM job was something they were not necessarily used to, but something they grew to like. I think they first came to be friends -- they teased each other, drank together, laughed and talked. They became very much at ease with one another. This, of course, paved the way for Jack to make his overture.
As the years passed, I think their friendship deepened, as did their passion for one another, even after the newness wore off. It was always good for them (SS). They became each other's habit -- comfortable, but contained within the various mountain ranges they explored. Jack loved Ennis and would have traveled around the world to get to him. Ennis, in my opinion, needed Jack's affection and love enough that he sometimes reciprocated. Deep down, Ennis was the needy one, and Jack met his needs. Jack made Ennis's life worth living, which is something Alma couldn't do. But, did Ennis love as much as Jack??? IMO, it wasn't untiil Ennis found the shirts that he really understood what love was and how much Jack loved him. Then, Ennis acccepted the fact that he did, indeed, LOVE Jack.