If you're the third in a triangle, you have a relationship with one person, who in turn has a somewhat similar relationship with another person. If you're the pig in the middle, you can just about cope but not when the two others suddenly come face to face.
If you're one of the other two, you either don't know about the other relationship, or if like Jack you do you have a tendency to discount it - "Oh, she doesn't really matter to him, I'm the really important one." Then you come up against the hard evidence, as Jack does, the wife and the two babies and the home made together and you're on the outside looking in. You see the person you think loves you looking awkward and embarrassed and not knowing how to behave in front of the two of you, and at that point all you want is to get him away and have him to yourself. You want to annihilate the third party. When she's not there, you can do so mentally at least.
Jack meets Alma just that once; next day he stays down by the pick-up, because although he's going to get Ennis to himself for a few days, this is where and to whom Ennis will return and he doesn't want to see her. Later all he wants to hear is Ennis saying that she doesn't matter to him as Jack does, but Ennis, torn between the two, can't be disloyal because he feels guilty and so defends Alma against Jack's implied criticism - and Jack thinks that means Ennis cares less for him than for Alma.
It's an impossible situation, as anyone who's ever been in it will know.