Why do you start and seem fear/Things that do sound fair?(1.3,51-52)Why do you think Macbeth responds this way?
The lines you ask about in Shakespeare's Macbeth follow the predictions by the witches that Macbeth will be Thane of Cawdor and king. Banquo notices Macbeth's reaction and says:
Good sir, why do you start and seem to fear
Things that do sound so fair?...
"Start" means flinch or recoil. Macbeth is startled. He is startled probably because he takes the predictions seriously, as Banquo does not, and probably because he is extremely ambitious and has considered this possibility before. At least these reasons are two possibilities.
Another possibility is that he not only has considered becoming king before, but already realizes what his becoming king would include--assassinating the ruling monarch, King Duncan. This might indicate why he flinches or recoils.
Notice how anxious Macbeth is to hear more. He wants explanations! As soon as the witches finish talking about Banquo's future, Macbeth says:
Stay, you imperfect speakers, tell me more.
I don't see an aside in Act 1.7, but his conversation with Lady Macbeth could be relevant to the question you ask. Macbeth decides not to assassinate Duncan, and his wife talks him back into it. This indicates he has reservations about killing Duncan, of course, but it may also indicate that he earlier flinches or recoils when the witches predict he will be king, because he has already considered the possibility of assassinating Duncan, and is already afraid of that possibility when he hears the predictions.
This might be a stretch, and it might be mere speculation. But I suggest it is a grain of evidence that contributes to the question of how ambitious is Macbeth before he hears the predictions, and how much responsibility Macbeth bears for his actions.
Incidentally, when Banquo asks why do you fear what seems fair, he is contributing to the fair is foul theme started by the witches and later echoed by Macbeth. And notice Banquo's words apply to Macbeth. Opposition is at play here, too, as Macbeth reacts in a manner opposite what Banquo expects.
If you read this to mean that he is simply startled, you can say that Macbeth is just confused about being addressed in this way. After all, he has just been addressed as two things that he is not. So it makes sense for him to be startled.
But if you think he is actually afraid, then this implies that he understands his own character and what will be in his future. It could be that he understands that he will stop at nothing to fulfill his ambitions. In this case, he would be showing that he is not really comfortable with his ambitious side.