Why do Macbeth and Banquo laugh about the witches' predictions?

At the very beginning of the play, Macbeth and Banquo are walking together after a victory in battle. These two men are confronted by three witches. These witches tell them a prophecy. Macbeth will become king, and Banquo's sons will become kings. While Macbeth takes this prophecy very seriously, Banquo simply laughs it off. He says, "Were such things here as we do speak about? / Or have we eaten on the insane root / That takes the reason prisoner?" (Act 1 Scene 3). After the witches share the prophecy, Banquo actually jokes that he and Macbeth are on drugs! Banquo is joking about the witches' crazy prophecy. Macbeth, however, does not laugh at all. At this point in the play, Macbeth begins to be consumed with ambition, and he desires to make the witches' prophecy comes true. Banquo and Macbeth have very different reactions to the witches' prophecy.