Jack had a tendency to bitch, IMO because he was a dreamer. He never wanted to settle, always imagined something better. A good example of this is when Ennis lost their food on Brokeback due to being startled by a bear, and Jack refused to settle for beans. Ennis was content to eat only beans, which shows us how in life he was more of a "that'll do" person. Jack wanted more for himself and Ennis.
But you're right, Jack never bitched about driving long miles to see Ennis because he was able to be with the man he loved, which was his joy. Jack, ever the dreamer, probably thought that Ennis would someday come around, and that was a goal worth driving 14 or so hours for. No need to bitch when working toward a dream. Hope this makes sense.