I think tantrum is a not exactly inaccurate, but is a bit oversimplifed. This is not the first time Jack has heard the word 'NO' from Ennis, but this is the first time that it wasn't complicated by the 'Ennis has a wife and family that depend on him' delusion. There would have had no doubts that Lureen/Bobby would have been well taken care of if Jack left, but there was no such family/monetary safety net for Ennis to know would catch Alma/his girls. That would have been enough to sustain Jack through the years -- Ennis was stuck and he couldn't just abandon his responsibilities, especially to his girls. When Ennis turns Jack away after the divorce, Jack can no longer deny the real truth -- Ennis isn't going to be with him no matter what. Jack was devastated, not like he couldn't have an ice creme bar before dinner, but like his whole world had just been yanked out from under him. I believe that anger was what got him to Mexico, but once he was there, the man who walked down that alley did not look anything but destroyed and desperate.