I would actually want to respond to this question by looking act Act III scene 1, which is when Macbeth plots the murder of Banquo. At this stage, he has already gained the crown, but the prophecies of the witches to both himself and to Banquo still echo in his head and haunt him, and he remembers well that the witches decreed that Macbeth would not be able to pass down the crown to any children and that Banquo's heirs would be king. Note what he says:
To be thus is nothing, but to be safely thus:
.... He chid the Sisters,
When first they put the name of King upon me,
And bade them speak to him; then, prophet-like,
They hail'd him father to a line of kings:
Upon my head they plac'd a fruitless crown,
And put a barren sceptre in my gripe,
Thence to be wrench'd with an unlineal hand,
No son of mine succeeding.
Macbeth clearly states his dissatisfaction with being King alone in this speech. To be "thus" or to be King is "nothing" because he fears that he will not be able to hold on to the crown and pass it on to his descendants. Notice the imagery with which Macbeth describes the trappings of power that are his because of his position of King. His crown is "fruitless" and his sceptre is "barren," pointing towards the way that being King is not enough for Macbeth.
--Macbeth, Act V, scene v