Great thread! Hey, Ruthlessly, you might consider changing the subject line to entice others.
I agree that it "takes two to two-step". The first time I saw the film, at the parting scene, I was incredulous: "What??!! Ennis, you're just gonna turn around a leave with a 'see you around'? and Jack, you're not going after him??" I just sat there with my mouth open, "What the f--- just happened?"
Not to get too shrinky, but each is set up for this:
Ennis was abandoned by his parents early on in life, and then is (unintentionally) abandoned by his sibling caretakers and becomes alone in the world. I think he expects to be alone, or to be left, and then does the abandoning himself.
In addition, he "abandons" his own feelings.
Jack clearly tells us of the rejection by his father: 'can't please my old man; never taught me a thing, never once came to see me ride.' (Many gay boys can relate to this). Perhaps he becomes tentative around Ennis ('I ain't queer') for fear of rejection.
And, they're both too young and unsure of themselves at this point to dare to try to be together. It's only later that Jack suggests it.