It is ambition that initially prompts Macbeth to plot the murder of King Duncan. Although it doesn't seem that he's ever dreamed of being the king before, when the Weird Sisters tell him that he will become king, Macbeth begins to dream of it. Although he is incredulous initially, when the sisters' first apparent prediction, that he will become Thane of Cawdor, comes to fruition, he starts to believe it might be possible for him to become king after all. Then, when Duncan names his older son, Malcolm, Prince of Cumberland and his heir to the crown, Macbeth considers how fervently he now aspires to rule the country, saying, "Stars, hide your fires; / Let not light see my black and deep desires" (1.4.57-58). He wants the stars to go dark so that no one will be able to look at him and see his new ambition for the throne.
Later, Macbeth recounts all the reasons he has not to go forward with the plan to kill Duncan, and there are many. However, he has one reason to stick to the plan: his "Vaulting ambition" (1.7.28). This ambition, then, is what has fueled his machinations so far.