In Act 1, Scene 5 of Shakespeare's Macbeth, the first third of Lady Macbeth's soliloquy is devoted to Lady Macbeth reading aloud a letter she has received from her husband.
In the letter, Macbeth reports his and Banquo's encounter with the three "weird sisters," or three witches. He particularly reports that the witches prophesied he would first be made Thane of Cawdor, then king. To prove that their prophecies are reliable, Macbeth next relates that once the three witches had vanished into thin air, messengers came from the king "who all-hailed [him] 'Thane of Cawder'; by which title, before, these weird sisters saluted me" (I.v.7-9). Macbeth's letter, according to Lady Macbeth's soliloquy, ends with Macbeth saying he wanted to report the good news to her so that she can take part in "rejoicing" (13). However, the rest of Lady Macbeth's soliloquy, immediately after reading the letter, shows some private thoughts of hers that serve to develop Shakespeare's major theme concerning the dangers of excessive ambition.
Lady Macbeth next states that she is afraid his nature is "too full o' the milk of human kindness/To catch the nearest way" (18-19). In saying "to full o' the milk of human kindness," she is saying a couple of things. First, infants drink milk, and in being nurtured with milk by their mothers or nurses, they are shown kindness. Hence, Lady Macbeth is first saying Macbeth's mind is too much like that of an innocent babe's. Second, in referring to kindness, she is also saying he is far too kind. The phrase "catch the nearest way" can be translated as meaning ascend to the throne as king as soon as possible rather than waiting. Hence, she is asserting that Macbeth is far too innocent and too kind to do anything dastardly or conniving that will win him the throne sooner rather than later.
Other claims she makes in her speech assert that he has ambition but not the drive to fulfill his ambitions; he is too holy and pure to become king; and that she knows he is willing to cheat to win what he wants.
All in all, as she speaks her soliloquy, she is planning to convince Macbeth to do something dastardly to win the crown, such as kill the present king. Since she is willing to stoop to such a low, immoral level to gain power, we can see that the soliloquy helps to develop Shakespeare's theme concerning the dangers of excessive ambition.