Why is Macbeth a classic? What makes it timeless?
The themes of Macbeth are clearly universal and compelling. However, it is worth reflecting that anyone could write a play about the corrupting effects of ambition and many people have. You or I could compile a list of the ten most universal themes in life or literature, write a play based on them, and we would have a play thematically richer than Macbeth. What makes the play a timeless classic and what makes Shakespeare an unparalleled poet is the resplendent brilliance of the language.
Consider the speech of the sergeant, a minor character who promptly dies in I.ii. The breathless intensity with which he describes Macbeth's victory over Macdonwald swarms (like the "multiplying villanies of nature") with magnificent images encapsulated in unforgettable phrases. Most audiences probably have no idea what "kerns and gallowglasses" are (they're differently armed soldiers) but the sound of the words is tremendous. In this one speech we have "brandish'd steel/Which smoked with bloody execution," "a rebel's whore," "valor's minion" and the thundering conclusion:
Till he unseam'd him from the nave to the chaps,
And fix'd his head upon our battlements.
Macbeth, before we meet him, is depicted in a series of electrifying phrases as a figure of unconquerable manic energy, ripping his enemy's stomach open and decapitating him.
One could choose a hundred passages and make similar observations. Dorothy L. Sayers selected the lines:
No, this my hand will rather
The multitudinous seas in incarnadine,
Making the green one red.
Sayers remarks "It is the thunder of the Latin polysyllables that makes the Saxon monosyllables so ominous and so terrific." This juxtaposition often occurs in Macbeth, but it is only one of a vast array of poetic techniques deployed as only Shakespeare can, which make the play a timeless classic.
It's an interesting question, given that this same exact query could be asked of so many works of Shakespeare (who, more than practically any other writer, tends to tower over the history of English literature and drama).
Macbeth is very rich, both from a thematic and a psychological perspective. Macbeth himself is a villain protagonist, whose character development is shaped by his ruthless ambition. We observe him as he seizes the throne through murder, but this is only the beginning of his bloodthirsty and tyrannical reign. Then there is his wife and conspirator, Lady Macbeth, who is racked with guilt for her role in her husband's crimes and is ultimately driven insane by that guilt. Macbeth then is a deeply psychological piece, addressing the psychological costs of violence.
Its range of themes is extensive. It is a political story, contrasting illegitimate tyranny with legitimate rule (which, for more modern sensibilities, can very easily be translated into a contrast between the abusive tyranny of Macbeth and the more responsible stewardship of Malcolm). It contains a discussion on hubris, with Macbeth becoming increasingly convinced of his own invincibility. It contains themes of fate and prophesy and also of the misinterpretation of prophesy. This thematic richness has certainly played a critical role in sustaining its reputation through centuries.
Macbeth deals with many universal themes:
1. The temptation that we all have to better ourselves in our career at the expense of others. Just as Macbeth it tempted to kill Duncan to advance his position, the temptation of "vaulting ambition" is prevalent in our cut-throat business practices.
2. What it means to be a man. The play defies machismo stereotypes and shows that a true man is not one who is ruthless and ambitious, like Macbeth, but one who is able and willing to sacrifice himself for the good of others, like Macduff.
3. The difficulty of judging others correctly. It shows how easily we can deceived by appearances, as was Duncan, and to some extent Banquo.
4. The devastating effects of guilt. Committing a crime has deep consequences on one's psyche and one's relationships with others.
5. The power struggles within a marriage, such as that as displayed between Macbeth and Lady Macbeth.
In short, Macbeth is a picture of human nature at its weakest and worst as well its finest--and human nature does not change. What Macbeth struggles with on this grand scale is something we all deal with on a generally much smaller scale. We are all tempted to want more than we should have or deserve, and sometimes we are willing to do something immoral or illegal to make that happen. We have been, as Banquo was, suspicious of our friends but want to believe the best of them, though people are generally not killed for having such suspicions. Some are willing to lose everything, as Macduff is, for the greater good. It's all part of the human condition.
The play also deals with the issue of fate and free agency. Does Macbeth really make his own decisions in this play or is he fated to act as he does? This issue comes up in many Shakespeare plays, with Macbeth and Romeo and Juiiet being the most obvious examples.