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.