In the beginning of the Shakespeare's tradegy, Macbeth, the relationship between the main character of the play, Macbeth himself and his Lady seems to be a very intense one and strongly based on mutual trust. They lean on each other under all circumstances and are true to each other no matter what happens. It also seems at first that there is a lot of respect between them, however in time Lady Macbeth takes a the part of the more dominant character of their relationship as it can be seen when she provokes Macbeth to kill King Duncan. She sees herself as a stronger person than her husband and that's why allows herself to decide what is best for both of them.
However the eventual murder of the King with all it's brutality and the overtaking of the throne destroys all the intimate feelings between Macbeth and the Lady. In the Act III, Scene II both of them change the roles they play in the live they share together.
When Lady asks Macbeth "...why do you keep alone?" he hides something very important from her. He doesn't inform her about the arranged murder of Banquo, the general of the King's army and was alone because he was just meeting with the murderers. This lie between them is what starts to tear them apart.
Lady Macbeth tries to comfort her husband by telling him that what was done was done and nothing can change that anymore, so he should not regret his deed. In return he tells her that he can't be at peace because the deed was still left unfinished and the threat is still out there. The prophecy of the witches "Sleep no more" comes true and macbeth says that actually the murdered king Duncan gets to sleep more...