I don't have the movie with me, but as I recall, Ennis was layin' back, looking up at the stars and smiling, and I imagine Jack was just ecstatic at the simple happiness of it all...just the two of them by the fire with the roaring water beside them. So when he said, "It could be like this, just like this, always" I thought he was referring to them being together like they were on top of Brokeback. They always used pronouns instead of verboten words like love, destiny, forever, and together. No instruction manual needed. They both knew what "it" meant.