Macbeth is a noble and respected soldier and will be rewarded for his efforts against the enemy. The witches (or weird sisters as they are called) prophesy that Macbeth will become king and Macbeth knows that this
"Cannot be ill, cannot be good” I.iii
basically foreshadowing what will follow. Rather than allowing the future to take its course, he tries to anticipate his glory and, along with Lady Macbeth, is impatient and willing to take any steps necessary to secure his position:
Macbeth cannot imagine that fate has more power than he.
By Act III, Duncan is dead and the prophesies seem to be coming true. Macbeth is king
"he is already namd and gone to Scone/ to be invested" II.iv.
referring to his "investiture" as king and Lady Macbeth gets to take her place next to Macbeth. Macbeth is however, paranoid and worries that, if the witches were right, his rewards may be short-lived and Banquo's sons will overtake him and become king in Macbeth's place. This is inconceivable and his reward is clearly not sufficient.