Why does Lady Macbeth view Duncan's arrival as "great news" in Macbeth?
After reading the letter from her "dearest partner of greatness," Lady Macbeth is in a state of elation that Macbeth has been given the title of Thane of Cawdor. However, she is also concerned that Macbeth may not have the brashness to go after the crown because although he has the desire he does not possess the cruelty and temerity to use any means necessary ("illness") in order to obtain this crown.
...wouldst be great,Art not without ambition, but withoutThe illness should attend it. (1.5.5-7)
...Hie thee hither,That I may pour my spirits in thine earAnd chastise with the valor of my tongueAll that impedes thee from the golden round,Which fate and metaphysical aid doth seemTo have thee crowned withal. (1.5.15-20)