Did you guys hear that supposedly in the script, after Jack said "sometimes I miss you so much I can hardly stand it", Ennis was originally gonna say "I know the feeling". But I guess they didn't want to include that. I think it would've been great if he said. It would give more proof to Jack that Ennis does love him.
Perhaps. The silence is quite ambivalent. Some people have interpreted this silence in a negative light -- as a refusal to acknowledge Jack's implicit declaration of love. I really don't know. That silenece can be interpreted both ways: as a refusal or an implicit acknowledgement. It depeneds on your perspective. For Ennis to have said something would have removed that ambivalence. Not exactly sure if this is good or bad.
I don't think the silence was a negative. We know Ennis is a man of few words. He had trouble saying what he feels. But in the very next seen we see Ennis and Jack asleep in the tent with Ennis's arm around Jack. I think he could always show Jack how he felt about him even if he couldn't say it.
For the record, I interpreted this silence as an implicit acknowledgement. 
Again, it depends on the viewer. I have read very good contrary arguments in this forum.
A lovely discussion, tpe & browneyes!
Ennis's lack of a verbal reply to ".. I miss you ..." allows us to view it as a silent acknowledge (if you're of the mind to read it that way, which of course I am!) and certainly plays into how most of the movie was presented. Not telling us everything, leaving room between what we are told and what we know intutively. There are other instances where they've deliberately pared down the speech in order to allow more room for the viewer to read into the moments what they "see" or "sense" from them instead of merely what they "hear", I think. We've mentioned before that there was dialogue in the ss for the motel scene that never made it to the sp or film, likely for that very reason. In a (great) film with great actors, vocalization isn't the only method for relaying the intent of the story. God knows, Jake and Heath conveyed plenty when words weren't spoken at all ...
So for ambivalence, also we have the look on Jack's face when Ennis is driving away for the last time. Could be that look of sadness and frustration on Jack's face is a signal that he's had all he can take; could be he's feeling as bad for Ennis as he is for himself and knows in spite of that he's going to keep coming back until Ennis tells him otherwise ... The "wish I knew how" part of the quitting, more ambivalence, more discussion here by those who read it one way or another.
