I think that Jake gave Jack a sadness we don't really see in the ss. He is what we call a Sunflower child. He had been brought up in an abusive environment but he still had the ability to want to love others. He tries, with Ennis, to go after a life he haven't had but still he isn't afraid to ASK Ennis for it
The rejection by Ennis, and the tears in the truck, are, to me, more and more, one of the important areas we are given, to untangle Jack Twist. Since I don't know how to multiple quote, have started with Esmelily and will go to each of the other posts, with apologies for picking and choosing, before trying to give my own take on what happened:
-MG: "Jack's tears in the truck.....showed how deeply Jack could feel, it was right after that we see a harder, maybe a more bitter Jack."
-MPJ: "...and every time his dreams fade away, it was like a crack in his self-trust wall..."
-AF: "From that point on, Jack was looking out for himself with much more caution."
-TPE: "I think the tears came because it was Ennis who hurt him."
-BT: "I think Jack had in his heart....a lot of things nobody got to see or understand."
-MG: "Jack was fighting back (in his own way)."
IMO, all of these are so very rightful, but am going with the last one, from MG, more completely. I don't think Jack went to Mexico for sex. I think he went there for revenge. I felt that the first time I saw the movie.
He was crying in the truck because his assumption that the divorce meant he and Ennis had a future was rebuffed. He wept all the way....to Mexico. When you are heartbroken and weeping, you are not....horny. Nor was there any smile or lust as he went down that alley. He was, and he knew it, being unfaithful, and this was his revenge.
Which leads me to another conclusion. All the Brokies who assume Jack was promiscuous after the events on the mountain (AP's hints notwithstanding) are, IMO, wrong. The film developed a different Jack (and AP has somewhat conceded, she knew Ennis, she didn't fully know Jack). And that Jack WAS faithful to Ennis until this tearful ride to Mexico. (Of course, I've also challenged the rodeo clown interpretation, too, saying it was trying for a friend, and not a pick-up, as that was just too much being read into a bar room conversation).
So, for me, Jack went from the sacred (sex with love) to the profane (male prostitutes), because the sacred had been profaned, when Ennis failed to reassure him. Of course, it couldn't have been worse timing, Jack pulling up just as Ennis was taking off with his daughters.
Even so, anybody who thinks that trip to Mexico was horniness, doesn't factor in the tears. It was revenge. Later trips....well, he'd found a place to go.
Please forgive my choosing the quotes that were convenient for this line of thinking. The writers would not necessarily agree with my conclusions.
They were all very good posts and everyone did so very well in doing what we rarely do....try to look for more to Jack than the standard take.