The music that accompnies this scene is really one of the great wonders of this movie, no? Once it starts when Ennis says "Then why don't you...", it never lets up. And it continues up until see Jack looking sad and tired as Ennis drives off. The emotional highpoint is made to coincide with the moment in the flashback scene when Ennis starts to hum to Jack. At that point, you are totally lost in a sea of conflicting emotions. Like the music, that moment has so much sadness and beauty.