I love this look also,because it reveals all what Ennis felt for Jack,but not always was willing to show,and less still with words.This is one of the greatest contradictions in him; keep on "nurturing" love inside Jack,and also inside himself,but never giving this love an opportunity to develop itself in a full sense...

That's beautifully said, mpj. That is indeed the crux of Ennis. He had only love for Jack, and no one else. He built up this love slowly over the summer until FNIT, then it exploded into SNIT, overflowing into the rest of the days and nights. But soon as they were back down the mountain, the lid came back on. And all he could do was to nurture it for the rest of his life, first the four years without Jack, then the sixteen years for a few times a year.
I wonder how long this would last after Jack's death. How long Ennis could hold on to this love, and not lose it without a constant reminder? Perhaps that's the significance of the two shirts and the post card. They are what Ennis knew he needed to be constant reminded everyday, so he wouldn't lose his way.
What you wrote (I highlighted in red) is really the essence of Ennis, the tightly clenched fist, holding on to the love he had for Jack, without ever letting go, except to knock Jack over.