John Milton was a prolific English poet who lived between 1608 and 1674. One of his more famous quotes is "calm of mind, all passion spent." The quote is from "Samson Agonistes" and is the final line of 14 lines of verse about a ruthless conqueror finding respite after a great military struggle. This quote can relate to many of Shakespeare's tragedies, but it certainly conjures images of Macbeth.
Throughout the play of Macbeth, Macbeth and Lady Macbeth are wrecked with guilt, anxiety and ambition. Macbeth describes this ambition in Act 1, Scene 7, before being interrupted by Lady Macbeth:
MAC: I have no spur
To prick the sides of my intent, but only
Vaulting ambition, which o'erleaps itself
And falls on th'other - (I.vii.25-28)
Macbeth's murder of King Duncan sets a chain of events in action that will cause Macbeth and Lady Macbeth to tirelessly fight to gain and sustain their power. In this moment, he describes how only "vaulting ambition" moves him to act further, but this ambition is speedy, reckless and unending.
Going back to the Milton quote, Macbeth does not achieve calm because his passion is not spent. The ambition of the Macbeths drives them to commit increasingly heinous acts, pushing them into madness. Perhaps, if Milton's quote is believed to be true, the Macbeths would have been able to rest if their passion had not been greedy and endless.