Why is Banquo's reaction to the witches' prophecies so different from that of Macbeth?Act II, scene I

For one, Banquo sees the witches for what they are--supernatural beings who play with the lives of humans.  He doesn't put too much stock in the messages, or perhaps his nature is not as ambitious as Macbeth's.  We never meet Banquo's wife, but it is possible that she is not the blood-sucking woman that Lady Macbeth is, and that her advice was more loving and sound.

For another, Banquo's prophecy was both better and worse than Macbeth's.  The witches told him that he is "lesser than Macbeth, and greater...Not so happy, yet much happier...Thou shalt get Kings, though thou be none."  So, from this we know that Banquo's nature is happier and greater than Macbeth's.  Perhaps not so in tune to ambition and power plays, but more honest, more loyal, more noble.  This is also what makes him a target for Macbeth later, since Banquo's children will become kings.  It does not set well with Macbeth that his throne will not be secure for generations of Macbeths to rule while Banquo and his son, Fleance, are flitting around.