In these lines, Macbeth reacts to the witches telling him he cannot be beaten until "Great Birnam Wood to high Dunsinane Hill...come against him."
His lines reflect his ego and his ambition because he doesn't even stop to consider the possibility he could be beaten. He immediately says, "That will never be." He is so blinded by ambition that he dismisses the prophecy and looks forward to becoming king.
He says that may the woods never rise until "high-placed Macbeth...live the lease of nature." Here, Macbeth is saying that he foresees himself as king, dying a natural death (likely of old age). His ego and ambition blind him to any other possibility.
