Why might banquo want to hide his thoughts from macbeth?

Banquo hid his thoughts to Macbeth because he too wonders about the prophecy that witches told about him producing a line of kings.  When the witches’ prophecy of Macbeth becoming Thane of Cawdor came true, this deepens his thoughts about it.  Also when the king dies and the prophecy about Macbeth becoming king comes true, this makes him even more suspicious.  Later on, Macbeth becomes worried of Banquo’s suspicion arranges to have both Banqou and his son killed. He manages to kill Banquo but Fleance is able to escape making him fearful that Fleance will threaten his throne.