My Lord is almost overused in this scene. This may be to show the new power dynamic that Macbeth has with Banquo as, now that he is king, he holds much more power compared to Banquo. As well as this, the power dynamic shows tbe control that Macbeth has on Banquo - his power almost compels Banquo to do his bidding. This way of addressing Macbeth shows that they are no long good friends as they used to be, now that Macbeth is king.