What a profound question - as are range of responses. For me, switching the focus from 'what makes Ennis tick' to why Jack is the way he is is part of the evolution of insights about BBM. We aren't given as much information about Jack, it seems at first, although maybe, in fact, we are given plenty..
I'm just remembering that when I first watched BBM with a friend of mine, she was very adamant that she didn't want to "be a Jack". She was involved with someone she saw as an 'Ennis', and what the film crystalized for her was that she did not want to play the Jack role. She didn't want to be the long suffering, patient one, putting up with constant avoidance and disappointment, never pushing too hard.
I understood what she meant, but I didn't like hearing her say it, because I think I preferred to idealize Jack. I think the way that Jack just kept loving Ennis, and trying to give him what he needed, even when it hurt him... when it kept hurting him, constantly.. is both beautiful and necessary to Ennis's development. Jack was Ennis's 'ministering angel'. His love had a sacrificial quality to it. And BBM is essentially Ennis's story, and for that story to unfold as it does it is necessary for there to be a Jack, and for Jack to act as he did.
But how do we reconcile that with a human Jack who.. well, maybe, held on too long, and neglected himself and his own needs? I think Jack is guilty of that... he cut himself off from part of what he needed, and maybe even denied the need itself, in order to be with Ennis. He forced himself to be satisfied with whatever little bits of life Ennis could share with him. And when dissatisfaction drove him to find consolation in other things and people, I think he ... well, I think he did it quickly and with his eyes closed, without acknowledging to himself what his life with Ennis was costing him, until that last camping trip when the shock of Ennis's cancellation of August forced the admission out of him in the form of anger.
Who acts like that? Maybe someone who doesn't have the self-esteem or the faith to try and find in this world what he really needs, or who, as someone has already said, has a problem with reality?
I don't know... but a long time ago, on IMDB, I posted something about Brokeback Mountain being a 'domaine' for Ennis – a 'domaine' in the Arthurian legend sense, a place removed from everyday reality, somewhere idealized, and somewhere that, having been there once, you keep trying to get back to. And someone posted in response to that idea – "Ennis' domaine was Brokeback.. Jack's domaine was Ennis". And, of course, wanting to idealize Jack, I didn't much like that idea either. It made Jack sound too screwed up, and I wanted him to be rational and right.
Could it be that love always has a f-ed up element to it? About 5 years ago on Valentine's Day I wanted to figure out the answer to that for once and for all, and I engaged the help of another friend of mine who is a therapist, and we had coffee and cake and discussed the issue. Why is it that love... the great loves we admire in literature, for example... always seems to lead people to act in ways that are not.. well, not really rational, or healthy-seeming? I don't think we ever resolved the question.
I read a poem, somewhere online, last year, and now I can't find it, but it said something to the effect of: "Do not give your heart to anyone who does not have the courage to cut pieces of their own soul to make a cloak to shelter you". That's pure Jack, to me. And yet.. well, the cutting pieces of your own soul does not really sound very healthy, does it?
Oh, I don't know. I guess what I really think is that Jack, having found companionship and love with Ennis, having found and recognized a person with whom he 'clicked' and matched, was afraid of never having that feeling again, if he let go of Ennis. I think he was afraid of his life feeling unreal and unconnected, to such a degree that he kind of manifested that very thing... a life that could never really come together for him, a life in which there was no true stable center, where even in death he couldn't be in one place.
But I think to act in any other way would have cost him the experience of loving so deeply and for so long. In the end I still think Jack WAS right..
ramble ramble and a bit OT... sorry!