After reading Macbeth's letter, Lady Macbeth is eager to divulge her plan for Macbeth's quick ascension to the throne of Scotland. She also knows that Macbeth is "too full of the milk of human kindness" to enact the deeds she imagines must be done for him to accomplish it. She remarks that he "has ambition, but lacks the illness that attends it" clearly indicating that she doubts Macbeth would agree to commit murder. This quote suggests that she needs to speak directly into the ear of Macbeth in order for her to influence his behavior. There is an awareness of her ability to "charm" him as his wife as well. Since Shakespeare read Milton, this quote alludes to the way the serpent whispered temptation into the ear of Eve in Paradise Lost, convincing her to take the forbidden fruit. Both the serpent and Lady Macbeth know exactly what to say to achieve their desired outcome.
Act I sc5. opens with Lady Macbeth reading Macbeth's letter in which he conveys to her the predictions of the witches and how one of their predictions, that he shall become the Thane of Cawdor, has come true. She next reads what he has written about the important prediction concerning how he will become the king of Scotland. Macbeth has innocently shared this news with his "dearest partner of greatness" merely to cheer her up and encourage her.
But lady Macbeth reveals to us that although her husband is an ambitious man he will not use any "foul" means to achieve his ambition. He is, as she rightly remarks, "too full o' the milk of human kindness" to take any short cuts to achieve his aim.
Macbeth can never ever hope to become the King of Scotland because Duncan has two sons, either of whom will become King after his death. Lady Macbeth clearly understands her husband's dilemma: 'how am I to legally acquire something which I am not legally entitled to?--"wouldst not play false/And yet wouldst wrongly win."
She remarks, that she will solve his dilemma by poisoning his mind with her evil plan of murdering Duncan. Unlike her husband she is made of sterner stuff and has no moral scruples because she is evil to the core and decides to convince her husband to murder the king.