What is Lady Macbeth's reaction to her husband's letter?
After meeting the witches on the heath in act 1, scene 3, and hearing that he will be king one day, Macbeth writes a letter to his wife, Lady Macbeth, to share the good news. We witness her reading this letter at the start of act 1, scene 5. In short, Lady Macbeth is excited about the news and is already thinking about how she and her husband can gain the throne.
She begins by reciting Macbeth's titles—Glamis and Cawdor—and affirms that he "shalt be / What thou art promis'd" (I.v.15-16). Lady Macbeth is certain that Macbeth will be king one day, so she's definitely bought into the truth of the prophecy.
However, Lady Macbeth is almost immediately worried that Macbeth will not be able to do what it takes to become king. She says that he is "too full o' th' milk of human kindness" (I.v.17), and that while may be ambitious, he does not have the wicked nature to achieve his goals. She imagines, though, that he would not regret doing a wicked deed if it earned him something he wanted. She concludes by hoping that Macbeth will be home soon so that she can "pour [her] spirits in [his] ear" (I.v.26).
Lady Macbeth believes that she can persuade Macbeth of the necessity of killing Duncan to gain the throne. She aims to remove "All that impedes" Macbeth from going through with the murder (I.v.28). After this soliloquy, a servant comes in to tell Lady Macbeth that her husband has returned, and the two begin plotting. It turns out to be true that Macbeth needs some persuading, but Lady Macbeth is ultimately not correct that Macbeth will not regret the actions they take to make him king.
Lady Macbeth is very gratified by Macbeth's letter, and she does not doubt for a moment that the rest of the Weird Sisters' statements will come true. She says, "Glamis thou art, and Cawdor; and shalt be / What thou art promised." However, she fears that Macbeth is too kind, too compassionate, and too loyal to Duncan to take the fastest path to the throne: killing the current king. She says, "Yet do I fear thy nature; / It is too full o' th' milk of human kindness / To catch the nearest way [...]." She admits that he is ambitious, but she feels that he lacks "the illness" which would compel him to privilege that quality over the others.
In light of this lack, she determines that she will need to plan and execute Duncan's murder herself, and when she learns that he is on his way to their home, she is terrifyingly ecstatic. Lady Macbeth is already resolved to regicide, as she refers to Duncan's arrival as his "fatal entrance" into their home. Like Macbeth before her, she addresses the sky and prays for darkness so that nothing and no one will be able to see her actions. She says, "Come, thick night, / And pall thee in the dunnest smoke of hell, / That my keen knife see not the wound it makes, / Nor heaven peep through the blanket of the dark /To cry 'Hold, hold!'" Such a coincidence helps to show us how well-matched this pair actually is for one another.
Lady Macbeth is very pleased with the witches prophecy that Macbeth will be king. Like Macbeth, she sees the immediate fulfillment of the title of Thane of Cawdor being given to Macbeth as confirmation that the witches prophecy about Macbeth sitting on the throne will come true. However, Lady Macbeth is much more ambitious than Macbeth. She sees the immediate obstacle of King Duncan and his sons. Macbeth is no where never being in line for the throne as things currently stand. Lady Macbeth doesn't feel that Macbeth has the ambition and decisiveness to take action and thus ensure the fulfillment of the prophecy. Lady Macbeth asks the gods to make her less of a woman and more like a man so that she can do what is needed (killing Duncan), which she doesn't think Macbeth will be able to do on his own.