In the beginning of this scene, Lady Macbeth reads the letter Macbeth, now both Thane of Glamis and Cawdor, has sent ahead of his arrival back home after battle. He tells her of the predictions of the weird sisters--that he would be king.
Lady Macbeth, knowing her husband well--that he is a loyal servant to King Duncan and a man of honor--realizes that he will not do what she feels must be done if she is to become queen anytime soon. She acknowledges his current titles, then says that he "shalt be / what [he] is promised," but, "yet do I fear thy nature; / It is too full o' the milk of human kindness / To catch the nearest way." In other words, he is a good man, kind and compassionate, a worthy thane (a Scottish lord). He is not the sort of person to "play foully" (as Banquo later muses) to get the crown.
Then she says something interesting: "Thou wouldst be great; / Art not without ambition, but without / The illness should attend it." Think about her choice of words here: she is acknowledging that realizing ambition requires a form of illness, a willingness to disregard the well-being of others, throw caution to the winds, and be utterly selfish--which Macbeth is not.