Your questions really made me think, Cameron. At first, I decided that the DE scene would be a happy one if it occurred out of context. But then I thought again. There IS something inherently sad about the scene. There are little clues. The soft grey muted colors. The half-extinguished fire, the smoke, the empty containers, the downcast eyes, heads, and hats, the muffled sounds of the horse. The "trembling" of Ennis's spurs. But, most of all, it's Ennis's rocking and humming of a lullabye. It's knowing that the words unsaid in this scene will remain unsayable and unspoken for their whole lives.
In Asian cultures, it is believed that part of the appeal of beautiful things is that they are fleeting and doomed. Beauty is ephemeral and elusive. Isn't that true? I thought that today as I caught sight of the full moon, shining in all its glory just a minute before it dipped below the horizon.
Ang Lee was in Europe one time attending a film festival and he was standing at a phone booth after the screening of his film near a line of women waiting to get into the restroom. They all had tears streaming down their faces. He said, "Just once I would like to make a movie that doesn't make women cry." His next film, Ride With the Devil, was a tearjerker just like Brokeback Mountain is!