Wow, so many fascinating ideas about what influenced Jack's upbringing and his becoming the person he is. I have some thoughts about it, too. Bear in mind, I'm a social worker but not a psychologist, so these are just my thoughts, not an assessment or anything.
A lot of what folks have said about difference and similarity rings true for me. When I think of Jack's development, I think about how both his parents influenced him, including the dynamic between Ma and Pa Twist. In some ways, like most of us, Jack is partly like his father and partly like his mother, in ways that are both wanted and unwanted. And like many of us, he recreates aspects of their lives that are familiar, even if unwelcome.
In the scene in the movie with his parents, his mother is the character that's most drawn from nowhere, since Proulx doesn't write about her that I can recall. To me, Jack's mother seems like a woman who is beaten down by life and most likely her husband. She is dignified, kind, compassionate, despite the likelihood of having lived a tough life with a hard, probably uncaring man, but she is not what she once was. So it makes me think that during his early life, Jack was probably exposed to a more outgoing, caring woman who he also brought low by a tyrannical father.
Jack's father we know a little bit more about, in particular how unwilling he was to share of himself with Jack. This selfishness was for his father probably a sense of competetiveness, although it didn't deter Jack from wanting to emulate his dad, i.e. the rodeo work. The urination scene strikes me of someone with an abusive, sadistic side, but to me, not necessarily sexual in nature. It was influential to Jack in a number of ways: it was a powerful memory of his father's penis, it highlighted a significant difference between them (probably one of many), and to a degree it may have been more sexual for Jack if as a child he experienced same sex attraction.
So what I see is what I imagine to be the dynamics between Ma and Pa Twist reenacted between Jack and Ennis. Jack is trying to live life, be happy, and love someone, despite the danger and tabboo of being with a man. His love interest is a man who is, to a degree, emotionally unavailable, highly disapproving, and has great difficulty giving of himself to Jack. I imagine that Jack's sense of optimism and hopefullness is an effort, probably not a conscious one, of trying not to fall into becoming like his parents. Sadly, because Ennis is so unable or unwilling to bend, to take a chance, this makes Jack more like his parents, although not the best of them: bitter (pa), despairing, beaten down but struggling to stay hopeful (ma).
Of course, this could all be projection on my part. When I think about my relationships, I sometimes think about how they're like my parents relationships and how they're different. I say to myself I want to have one that's different, or at least doesn't emulate some of the worst parts. Maybe that's why fan fiction seems to popular right now, this wish that things didn't turn out the way they did, that in some way Jack lived, Ennis changed and they lived happily after ever. Hmmmmm.
But on another note, I really hope that you don't leave the thread Daniel, you've always been so good in that role!
Juan