Macbeth has been toying with the notion of becoming king by killing Duncan, as we know from Lady Macbeth's reading his lettter aloud at the beginning of scene 5. We can assess his emotions accurately from what he writes in that letter.
When I burned in desire to question them further, they made themselves air, into which they vanished. Whiles I stood rapt in the wonder of it came missives from the King, who all hailed me "Thane of Cawdor," by which title, before, these Weird Sisters saluted me to the coming on of time with "Hail, king that shall be."
So Macbeth was experiencing a variety of emotions. He burned with desire and was rapt in wonder. No doubt he was also thrilled by the witches' predictions and then shocked and delighted when Ross greeted him as Thane of Cawdor. But in addition to these other emotions, Macbeth is suspicious. He blurts out, "The Thane of Cawdor lives." What he is thinking is that this is too much of a coincidence. Is somebody trying to trick him into revealing his secret ambition and his inchoate speculations about the possibility of turning it into reality? So suspicion and apprehension can be added to all his other emotions, all of which he must conceal, and most of which he has never experienced before.
When he asks, "Why do you dress me in borrowed robes?" he is trying to hide his feelings from everyone present by making a joke of the question. There is a big change in his tone of voice and his facial expression between "The Thane of Cawdor lives" and "Why do you dress me in borrowed robes." The modern American equivalent of his question might be something like, "Come on! You're kidding! What's the gag?"
Macbeth is going to have to do a lot of acting within a short time. He has to act like the loyal and loving servant of King Duncan when he and his wife welcome the old man to their castle, and he has to act shocked, innocent, and incredulous after Macduff discovers the body of the man Macbeth has murdered. Since Macbeth is being played by an actor, this player will be called upon to be an actor acting like an actor.
Lady Macbeth's soliloquy after reading the letter aloud indicates that, among other things, this shrewd, calculating woman has doubts about her husband's ability to play his multiple roles convincingly.
What a great scene! Macbeth gets some very interesting news here. He's told by the witches that he will be Thane of Cawdor and later King of Scotland. But, his emotional state does not jump right to excitement as we might expect. He is suspicious of the witches (as he should be, for he knows, along with the people of his time, that witches could do the devil's work and were evil creatures); he is in disbelief that these predictions could be true. But the witches have made an impression and, later in the same scene, when Macbeth is informed he is indeed now Thane of Cawdor, he begins to believe the greater prediction of King could come true.