Answer: Because Lady Macbeth is not sick, she is feeling guilty because of the murders she and Macbeth committed. The doctor can do nothing about it.
Explanation:
Upon hearing the Three witches' prophecy that he would become a king, Macbeth and his wife devise a plan to make it happen. Macbeth murders king Duncan and Banquo, which makes him haunted by guilt. He does become a king, but his life is far different from what he hoped for. Even Lady Macbeth, who had the initiative at the beginning of the play and convinced Macbeth to 'do the deed', gradually becomes weak and frightened. She starts to sleepwalk, see blood on her hands and claims that she will never be able to wash it.
In <em>Act 5, Scene 3</em>, Macbeth demands that the doctor cures her of her delusions. This demand is ironic, in that it is not possible. Lady Macbeth is simply consumed by guilt and is not feeling well because of it. No doctor will ever be able to cure her from her guilt. Macbeth is concerned both because the enemy is getting closer and because of his wife's state of mind, therefore, he cannot accept that his problems are not to be solved over night. He blames the doctor for being unable to help Lady Macbeth, while, in fact, it is only their fault that their plan did not meet their expectations.
