For answering to this question I was surprised to start thinking about their last day on BBM.Jack was not worried or upset to leave as he didn't need BBM to love Ennis,he knows his feeling about him, BBM or not.His desire to joke,his thoughts to have another summer together the next year.The Ennis'shirt "theft" was made after the punch,yes,but I don't think it means that he thought it was unlikely that they met again, if it was we haven't had the "Twist, you guys wasn't gettin' paid to leave the dogs babysittin' the sheep while you stem the rose" scene with Aguirre.It's just one of the many things that shows us his love for Ennis.
I guess the anger he throw away in the "I wish I knew how to quit you.” scene begins when Ennis said "I guess I'll see you around, huh?" He looks so surprised by Ennis been so distant.I wasn't surprised by the four years without trying to known where Ennis was and He wouldn't have looked for him If he wasn't so in love with Ennis,do you remember?"he had been riding more than bulls, not rolling his own".He didn't looked for Ennis only for sex. I could go on listing the Jack's disappointmens for many times asking "Why he didn't give up on Ennis before?" their meeting after Ennis' divorce...another bad surprise for Jack,what a let-down for him! How to react!...well,we know how he reacted.I guess he didn't give up on Ennis in the end, but he wouldn't have tried to see him anymore.He'd have waited for an Ennis move.
It's easy for me to understand Jack feelings at this point, but, as Cassie said,I don't get you Ennis Del Mar...Yes,I understand Ennis feelings on the BBM last day, I can also understand the punch and the cold goodbye, especially after his breaking down ...he couldn't go back on BBM the next year,he just get married with Alma and he takes "about a year to figure out it was that I shouldn't’t a let you out a my sights. Too late then by a long, long while.” He couldn't knew how to find Jack, so what could he make?.But I can't understand his "inertia"...I can understand his fears but how could they be stronger than his love for Jack? If Jack were not died, what would he have made? I don't know how to answer to this one.