Having spent a night together with Mattie without the troubling presence of Zeena in Chapter Five, Ethan wakes up the next morning in Chapter Six with a feeling of happiness and joy. Although, as he reflected at the end of Chapter Five that he had not even touched Mattie's hand, as he had desired, still he had greatly enjoyed his evening and the intimacy that they shared. Note what we are told at the beginning of Chapter Six:
He did not know why he was so irrationally happy, for nothing was changed in his life or hers. He had not even touched the tip of his fingers or looked her full in the eyes. But their evening together had given him a vision of what life at her side might be, and he was glad now that he had done nothing to trouble the sweetness of the picture. He had a fancy that she knew what had restrained him.
This quote therefore tells us that Ethan was so happy because he had had a "vision" of what their life together could be like, sitting together in marital happiness. He also feels that Mattie knows what was going through his mind and shares the same desires. However, at the back of Ethan's mind is always the knowledge that this is but a temporary state of affairs and that his wife is due back that day, which will put to one side his imaginative flirtations with the idea of living with Mattie permanently.