This is probably one of the most intense elements about the epic.  It is one of those issues where discussion and discourse might end up revealing more thoughts and paths, but little in way of definitive answers.  Rama's harsh treatment of Sita can be seen in a variety of ways.  It is not merely in words, or in how he speaks to her, as it is his overall demeanor towards her once Ravana is defeated.  It is this treatment that becomes one of the most debated elements about Lord Rama's actions upon the defeat of Ravana and in connection to his wife.  One would be that Rama is the ultimate upholder of dharma or duty.  His citizens believe Sita to have been unfaithful while she was in Lanka.  In deference to his citizens, Rama assumes the ultimate in leadership dharma, sacrificing his own sense of personal for the public and the responsibility of a leader.  Another point of view might be that Rama saw his duty as having to rescue Sita as the dharma of a husband/ king.  His treatment of her is one in which he looks at her as a responsibility, something that was required of him as a demand of duty.  Finally, some might argue that his treatment of her in such a harsh light was to actually prove her sense of chaste virtue to all.  In this thought process, Lord Rama could not immediately support Sita in the face of his citizens' doubts and questions.  In response to this, he distances himself from her in order for her to walk through the fire and demonstrate her own virtue to all, something that places her above reproach and above any sort of connection to other human beings.  I think that in this, one sees that his treatment of her is one way in the short term with a longer term vision in play.