In Macbeth, what is the relevance to the theme of ambition in Banquo's reaction in I.iii.51-52?
In this scene, the witches greet Macbeth with praises that he is Thane of Cawdor (a title which has not yet been bestowed upon him) and that he will become king as well. Banquo, noticing that these women who "look not like th'inhabitants o'th' earth" might be figments of his and Macbeth's imaginations or they might be supernatural creatures, perhaps with the ability to make predictions about the future.
After they make such predictions for Macbeth, Banquo first asks Macbeth why he seems startled by predictions which seem to bode well for him. Then Banquo recognizes that Macbeth is "rapt" (enchanted) by these predictions. Now Banquo wants to know if they can make predictions for his own future.
You greet with present grace and great prediction
Of noble having and of royal hope,
That he seems rapt withal. To me you speak not.
If you can look into the seeds of time
And say which grain will grow and which will not,
Speak then to me, who neither beg nor fear
Your favours nor your hate. (I.iii.53-59)
Macbeth is enchanted by their predictions and this is the first indication of Macbeth's ambition. Banquo is simply asking about his own future. Note that he says he does not ask (beg) nor fear what predictions they might have for him. Banquo is not interested in ambition or praises; he's simply curious about his own future. This is a healthy curiosity which we will learn, later in this scene, is a part of Banquo's equally healthy skepticism:
And oftentimes to win us to our harm
The instruments of darkness tell us truths,
Win us with honest trifles to betray's
In deepest consequence. (I.iii.121-24)