This scene is is soooo sad, poor Jack,
so sure that loving Ennis and being with his one true love is worth any sacrifice, no hesitation.... I don't know how Jack survived this disappointment (disappointment doesn't even begin to describe). damn you Ennis!
Someone asked Jake in an interview (going by memory here,
) : "How did Jack die?" or something similar, and Jake answered (I am paraphrasing): "Jack starts to die the moment he realizes that he will never have the sweet life with Ennis". I think this is the moment, and I don't know how Jack ever had the strength to keep going back to Ennis for another eight years.
J