I feel that this assertiveness is true of Jack in almost every case except for with his Dad. I don't have any great proof or reasoning for this, but I feel like Jack would be pretty intimidated by his Dad.
I agree that Jack can be every bit as assertive as Ennis, and then some. But their differing degree of self-confidence gives their assertiveness different triggerpoints, it seems.
Ennis has next to no self-confidence at all. He takes every slight and possible insult much more personally as a result, and either stews on it in hurt but seemingly unaffected silence, I think - or explodes in uncontrolled rage if there's a sexual connotation involved.
Jack on the other hand has a good deal of self-confidence, but also a very disarming ability to laugh at himself and to not take himself too seriously. It takes quite a lot for him to become overtly assertive - he knows and believes in his own worth, he doesn't feel the need to get up all in arms every time someone steps on his toes. (The Thanksgiving show-down in my opinion is partly due to his sense of obligation towards Bobby. The boy needs a father to be proud of; - and moreover he needs to be "saved" from having LD as his (only) role model. If not for his son's sake, I'm not entirely certain Jack would have bothered with the tension of having an open confrontation with LD, no matter what his father-in-law might think to say to provoke him.)
Now, how Jack ever got that healthy self-confidence is a bit of a mystery to me. Growing up on that lonely farm with *that* unsupporting, ever displeased and negative father? (I'm taking Jack's statements about his father at face value in this - how his father never came to see him bullriding, never taught him anything, how he couldn't ever please his father, no way.... The one meeting we have with old man Twist certainly doesnt do anything to contradict Jack's tales of his father. I'm also keeping the short story's tale of child abuse out of the equation since it's not in the film.) Jack's mother must have been the main force and supportive constant in Jack's life and development for him to become the character we get to know. But inevitably I think that whenever grown-up Jack had dealings with his father, he'd find it hard to shake the feelings of the boy who never measured up to his father's expectations, of being seen as inferior, not quite good enough - all that his father instilled in him from childhood. Plus, there must have been a strong feeling of resentment. I think much of Jack's normal self-confidence evaporated where his father was concerned. I wonder if he'd bothered going back home to visit if his father had been a widower. I guess not. But perhaps he'd still be trying to impress his father into approving of him at last - if he couldn't do it through bull-riding, perhaps he could do it through eventually "whipping the farm into shape".
(It's awfully late here, so apologies if this post is even less coherent than usual.)
