One of the key themes of the novel is that of cause and effect or perhaps fate, the idea that characters cannot escape the events that are set to happen to them and that one event follows another in an inescapable sequence. The inevitability of all the events following the meeting between Tess and Durby-Field is a good example of this theme.
And this theme is inextricably tied to the idea that these same events are caused by or linked to aspects of the main character's personality. Tess' incredible sense of morality, responsibility and her willingness to work hard to rectify difficult situations leads her into many of the terrible events that occur. It also serves to highlight the benificence of her character because of the poor character of those around her including her parents and her siblings. Tess of course tries to protect their reputation and in so doing she is often brought into more terrible situations.
Lastly he brings up the theme of her good nature being connected to the beauty and wonderful qualities of the place of her birth and young life, Wessex. Hardy does this by describing in detail the wonderful things present in Wessex and how they've helped to shape Tess.