I agree with Lee that the wonderful, fateful summer on the mountain is the most joyful element in both story and film. Ennis and Jack got to know each other, themselves, and got to be who and what they really were for that brief, bracing spell. Their stint on the mountain very much haunted the rest of their lives--note how they try to recreate it with all those fishing trips.
There is a part of me, though, that believes that all sentient experience has its roots in a mysterious Cosmic Joy, so that, seen through this lens, even the saddest, most harrowing parts of the story have a spark of joy in them. The very fact that Jack and Ennis found true love, the greatest, most precious treasure that can crown a human life, is cause for any of us to feel exceedingly grateful, enduringly satisfying joy.