After Ennis's threat, it was like Jack said OK you SOB, here it is. Jack was pleading for his life, Ennis's life, "look at what you're doing to us". But did Jack really understand what Ennis sacrificed? And his sacrifice was so much more profound. Ennis gave up his entire life, for the both of them. He may have said it in clumsy way, not eloquently, "It's because of you I'm like this", "I'm no where." He breaks down. Every move Ennis ever made in life, was gear to being with Jack. His fears of what would happen to them both was always a factor, but he lived within the limitations of that, but his love and need for Jack was so over whelming that he couldn't let him go, nor would he.
Jack had it better off, his money gave the confort live a little better on the mountain. better tents, better food, shooting elk for the pleasure of the hunt, and not for survival. Ennis was still carry his belongings in sacks. Ennis was boarding the horses, but how much do you think that cost him, a lot, or was he still bartering so he can keep the horses, so they can ride together. Was he living in a nice house, like Jack, or a rundown shack? He made connections with no one, his whole being belonged to Jack.
With all of this, Jack is seeing Randall on the side. If Jack just had a glimpse of Ennis's life would he have understood just how much this man loved him?