We want moral consistency. We want a fixed viewpoint. We want to know who to cheer for, who to hate. We want goodies and baddies. And he refuses to oblige.
In Measure for Measure it throws us off balance that the Duke is both Godlike and creepy, that Isabella is both heroic and self-centred. We want the wrinkles smoothed out. We want them to be one thing or the other.
The same thing happens in All's Well That End's Well. Helena is both noble wife and obsessive stalker, Bertram is both the perfect Renaissance gentleman and an utter shit- and it confuses us. It makes us angry. We wriggle out of our discomfort by calling the play a failure.
The so-called Problem plays are the ones that take this characteristic of Shakespeare's art to its most unsettling extremes- It doesn't help that they're billed as comedies- but it's to be found everywhere. The lovable Hamlet is cruel to Ophelia, callous in his treatment of Rosencrantz and Guildenstern; Goneril and Regan have right- or at least commonsense- on their side; Henry V is both national hero and cold-hearted politician; Falstaff is utterly unscrupulous but also the personification of merry England. There's a debate to be had about all of Shakespeare's major characters- are they this? Are they that? Each one is a drama within a drama, a puzzle on legs. It's the reason why actors still want to play them, why- after 400 years- the plays are still so stageworthy, so much alive.
Shakespeare is not a moralist. The waters are always muddied. His plays have nothing to teach us- except that this is what life is like.