Transformation is a very big deal in this play, which isn't so surprising because one of Shakespeare's main literary sources is Ovid's Metamorphoses. In the third act of A Midsummer's Night Dream, Puck uses magic to turn Bottom's head into that of an ass (a.k.a. donkey). Although this is the most obvious example of transformation, it's just one of many. Throughout the play, characters undergo physical and emotional changes – they fall in and out of love and change their minds about their friendships and the world in which they live. The natural world of the play is also subject to transformation – night turns into day, darkness turns to light, the moon waxes and wanes, and so on.
When Titania falls in love with an "ass," the play reminds us that love can transform even the smartest person into a blind fool.
Shakespeare's magic love juice is a lot like Circe's magic potion in Ovid's Metamorphoses – both concoctions have the ability to transform the desires of victims.