A river, undisturbed and untouched, will naturally be beautiful, peaceful and serene. The wind may stir its surface once in a while, but only a big disturbance will taint its waters. Whenever I try to illuminate the themes of Tess of the D'Urbervilles in my mind, I see a great big oil tanker dumping oil on an otherwise untouched surface of water. Thomas Hardy, I believe, meant to bring rise to many conflicting subjects, through his elaborate portrayal of nature and the human condition.
At the center of all these themes is the war between nature (old ways and every other kind of purity represented) and modernism (everything otherwise conflicting with nature). Is Tess a victim of society, or is she a victim of herself? Tess dies for several causes, I think. She has spent her entire life fighting against the social dogma, disregarding what she ought to do for what she thinks is right. But in this she has also wagered a war against herself and her own morality. She gave up her own temporary happiness with Angel for the sake of being honest and maintaining some purity. They could not be together because of social proprieties; they could not be together because Tess was a victim of nature itself.
She made sure that in her life, everything was out and open; her approach paralleled nature. Unfortunately, Tess could not control the new agricultural system and modern ways of life popping up around her. These are the hardships of Tess and the human condition.