Why does Macbeth say, “if chance will have me king why, chance may crown me without my stir”?

It's Act I, Scene III and Macbeth is marveling at the fact that two of the witches' prophecies have already come true: he has become both Thane of Glamis and Thane of Cawdor. Macbeth is seriously starting to believe that the most important of their prophecies will also come true, namely that he will one day be king. At this stage, however, Macbeth isn't seriously contemplating the murder of Duncan. He's thinking about it, alright, but outwardly, at least, he remains loyal. Indeed, it's important for us to bear in mind just how hard Lady Macbeth has to push her husband to get him to commit regicide.
And this leads us on to the meaning of the above quotation. Macbeth believes that fate has already given him two titles without his needing to do anything. So, he figures, why not the third? Why not the most important title of all, that of king? Macbeth is trying to talk himself out of the necessity of killing Duncan. If Macbeth is fated to be king, then why not just let it happen instead of actively trying to do something about it?