Shakespeare Quotes: What fools these mortals be
In Elizabethan folklore, Puck (a.k.a. Robin Goodfellow) was a household sprite who, depending on his mood, played annoying tricks on people or helped them out with their chores. This explains why Shakespeare's Puck brags to us about all the times he's been a pest to local villagers by sabotaging vats of ale and ruining the batches of butter that housewives spent all morning churning.
So we know when Puck is given the task of distributing love juice, that things might not go as planned. After turning the young lovers' world upside down, Puck still blames the fact that the lovers are acting crazy on the fact that they're humans. Forget his melding or love juice. It's all this crazy little thing called love. And human nature, evidently.
Even though Puck messes up, we love him for it. He has a way of entertaining us. After all, it's this mistake with the love juice that sets the whole plot in motion. And ensures that the right lovers end up together by the end of the play.
By sending the lovers into chaos, Puck also makes sure that we, the audience, have a good time as well. In this way, Puck is also a kind of "lord of misrule" figure—you know, a guy who was appointed to reign over carnival festivities, which included drinking, eating, and raucous theatrical productions.
It's fitting, then, that Puck should close the play by delivering the Prologue. He is also the only character with the credibility to tell the audience that he knows the play is unreal, like a "dream," and he promises that, if we didn't like the play, he'll soon make it up to us with another one.