After the reunion, there was no way I believe Ennis would have let Jack get away again. After say that, It's a miracle that Jack stayed with Ennis. For all those years after that Ennis's fears, his struggle with his own sexuality, his insistence in dominating Jack, there was really no reason in the world for Jack to stay with him and to love him the way he did.
Their need for each other, emotionally, sexually, and even an almost spiritual connection between them, bound them. These things sustain a love on a mountainside for twenty years. And in that tells us about love and commitment to someone you may consciously or unconsciously need more than just air to breathe.
But all in all, there was subtle play for power, to keep what they had for each alive, but also slowly killing what they need most, each other. The final confrontation was example of that. Jack was fed up with Ennis, felt Ennis was being cruel for giving less and less of himself, Ennis struggling with his own sexuality, his need for Jack, and fear of losing him to the point of threatening Jack's life was so conflicted he broke down in Jack arms, there was a lot emotions going on. More then just surface, but to the core of each man. Who lost control than and why?