The Reason We Deny The Role of Luck (via Time.com)
At any point during a Shakespeare tragedy should we just kind of look at it and think, "Dang, you know, that was just really unlucky"? How about Polonius being behind the arras in the first place? Sure, it was pretty impulsive of Hamlet to just go all stabby all of a sudden, if he'd done that literally any other time when somebody wasn't back there, the play would go totally differently.
The article linked above asks why we feel obliged to pretend luck doesn't play a factor. Luck suggests that even if you don't do the right things, you can still come out ahead (people like to cite Bill Gates, college drop out, as a great example here). Or, that you can do everything right and still one day tragedy strikes and you lose everything. It's hard to accept that sort of randomness, because it acknowledges a complete lack of control. If I choose a certain path, I want to expect that certain things will happen. If an unexpected thing happens, my brain wants to go back and create a new path that I must have taken to get myself to that spot.
Personally I believe in the theory that says, "At any given time, you are the sum total of your experiences and decisions up to that point." I always take issue when people say something like, "I'm happy with my life, I just wish that X had been different." You can't have it that way, because if X had been different, then everything that came after X would also be different.
Luck, therefore, is part of the definition -- a thing happened at a certain time because of conditions that all your previous decisions got you into. Luck is basically the uncontrollable bit. Sure, Hamlet decided to go to his mother's room, get all upset, and murder the tapestry. But nothing he did was responsible for putting Polonius back there. Sure, sure, you could argue that the whole play-within-a-play, which deliberately pissed off Claudius, set Polonius into action, but ultimately Polonius has free will as well that Hamlet does not control.
I guess the whole point is, does the tragic flaw exist? Or is it just a construct we put in place after the fact, to rationalize what is ultimately just a series of uncontrollable events, making your choices like waltzing through a mine field and hoping your next one doesn't blow up in your face.