Lady Macbeth is thrilled that the witches have predicted her husband’s success, but she is annoyed because she does not think he has what it takes to get there.
After the witches have prophesized that Macbeth will be the next king, even though Duncan names Malcolm his successor, Macbeth writes a letter telling Lady Macbeth what happened. She is happy, but she quickly realizes that her husband may not have what it takes to grab the throne from Duncan.
Yet do I fear thy nature;
It is too full o’ the milk of human kindness(15)
To catch the nearest way. Thou wouldst be great;
Art not without ambition, but without
The illness should attend it. (Act I, Scene 5, p. 19)
In other words, Macbeth has ambition but is just too nice to make anything of it. He is not going to do what it takes to become the next king. He will not be driven with an inner "sickness" to do anything to get ahead, even to kill Duncan.
Of course, Lady Macbeth knows her husband. Since he won’t do it, she will. She plans to take full advantage of the situation and develops a comprehensive murder plot, complete with a frame-up job. She may know her husband is a valiant warrior, but she also knows that he is not compulsively driven toward gain the way she is. She expects that she can manipulate him though.
Lady Macbeth knows that her special skills will be needed as soon as she reads the letter. She goes into an excessive ambition, planning everything that might go wrong. She wants Macbeth to be king, and she knows he wants to be king. All she has to do is push him. The letter is our first introduction to Lady Macbeth, and we realize that she is ruthless and ambitious and more bloodthirsty than he is.
The letter outlines everything that has happened to Macbeth, including the predictions by the Witches. Lady Macbeth is exited by the news, but also wonders if her husband is too soft, or kind-hearted, to be able to do what must be done to insure that he becomes the King of Scotland. She determines that she must be his backbone and make certain that he is able to carry through with Duncan's murder.