Macbeth is unravelling, and in this scene, he is emotionally overwrought and on edge. As the scene begins, he hears dire reports about Malcolm marching on him in concert with the English army. Macbeth dismisses the reports and insists loudly and stridently that nothing can defeat him. He reminds himself of the witches' prophecies and relies on these prophecies all the more heavily as the reality surrounding him signals that he will be defeated.
A servant comes and tells him the English army is 10,000 strong. Macbeth chides the servant, brags of his strength and refuses to accept what contradicts the prophecies, getting ever more wound up as the scene progresses. Privately, he admits to Seyton, however, that this coming battle will decide his fate: he will either win the battle or be unthroned. He realizes he has sacrificed all the comforts of old age to his ambition: he has become a tyrant who everyone lies to and fears; he cannot expect to be surrounded by "honor, love, obedience, troops of friends."
We thus see Macbeth in a frenzied mood: on one level, he realizes the end has come, but on another level, especially in front of his troops, he still clings in a desperate and defiant way to the witches' prophecies because he has nothing else to hang on to.
Dramatic irony operates in this scene. The audience has been privy to Hecate, the head of the witches, having an angry conference with the hags, telling these witches she will use the hope raised by prophecies to mislead Macbeth and lead him to his doom. Macbeth doesn't know his fate has been sealed. The audience does not yet know how the prophecies will play out to ensure Macbeth's doom.
Macbeth is so confident because of what the witches told him. He seems to think that he is untouchable and that none of those crazy prophesies can come true. The woods are not going to come marching up to his castle, and every man is woman born. He remembers and recites, "Fear not, Macbeth; no man that's born of woman Shall e'er have power upon thee."
His overconfidence is his downfall. He is unable to see his fate as he should. Even the declining mental health of his wife doesn't seem to bother him. He tells the doctor to give her the cure, then walks away. He seems to assume all will be well for him.