EGEUS With cunning hast thou filched my daughter's heart, Turned her obedience (which is due to me) To stubborn harshness. (1.1.137-139)
According to Egeus, Lysander has used his charms and "cunning" to transform his once obedient daughter into a stubborn girl.
HERMIA Before the time I did Lysander see Seem'd Athens as a paradise to me. O, then, what graces in my love do dwell That he hath turned a heaven unto a hell! (1.1.209-212)
It's clear from Hermia's ranting that love also has the capacity to change the way we view our surroundings. According to Hermia, her love for Lysander has transformed her own home into a nightmare world.
THESEUS Hippolyta, I wooed thee with my sword And won thy love doing thee injuries, But I will wed thee in another key, With pomp, with triumph, and with reveling. (1.1.17-20)
Theseus believes love has the transformative power to change him from Hippolyta's enemy to her lover and to change Hippolyta from an Amazon Queen to a happy wife.
HELENA He will not know what all but he do know. And as he errs, doting on Hermia's eyes, So I, admiring of his qualities. Things base and vile, holding no quantity, Love can transpose to form and dignity. Love looks not with the eyes, but with the mind; And therefore is winged Cupid painted blind. (1.1.235-241)
Here, Helena admits that she loves many of Demetrius's "base and vile" qualities. It seems that love has the capacity to blind us (figuratively speaking) to the truth. This idea comes up again when Titania literally falls in love with Bottom—a "base and vile" creature.
Act 2, Scene 1
OBERON Fetch me that flower; the herb I showed thee once. The juice of it on sleeping eyelids laid Will make or man or woman madly dote Upon the next live creature that it sees. (2.1.175-178)
Oberon tells us that, when the magic love "juice" is sprinkled into someone's eyes, it causes the person to fall instantly in love with the first creature he or she sees. Shakespeare seems to have borrowed this concept from Book 14 of Ovid's Metamorphoses, where Circe uses a magic potion to transform men into beasts.
Act 3, Scene 1
QUINCE Bless thee, Bottom! bless thee! thou art translated. Exit
BOTTOM I see their knavery: this is to make an ass of me, (3.1.120-123)
After Puck has "translated" Bottom's head into that of an ass, Bottom becomes the butt of Shakespeare's biggest joke about transformation. Clueless that he's been transformed, Bottom declares that his friends have run away from him in fear because they're trying to "make an ass" out of him (3.1.16). Shakespeare probably got the idea from Apuleius's Golden Ass, a hilarious and ancient story about a guy who's turned into a donkey. Bottom's conversion is also key to the play's theme of transformation, a concept Shakespeare borrowed from Ovid's Metamorphoses.
TITANIA Out of this wood do not desire to go. Thou shalt remain here whether thou wilt or no. [...] And I will purge thy mortal grossness so That thou shalt like an airy spirit go. (3.1.154-155; 162-163)
There's a dark element of coercion here, where Titania informs Bottom that he'll remain in the wood with her, regardless of whether or not he wants to. What's more, Titania is ready to use her magic to physically transform Bottom's mortal body into that of an "airy spirit."
TITANIA I pray thee, gentle mortal, sing again. Mine ear is much enamoured of thy note, So is mine eye enthrallèd to thy shape, And thy fair virtue's force perforce doth move me On the first view, to say, to swear, I love thee. (3.1.139-143)
Bottom has literally been transformed into an ass, but here it's obvious that Titania has undergone a transformation as well. Oberon's love juice has turned the once-feisty and intelligent queen into a silly, love struck woman with no ability to judge appearances. Though we know that Bottom's voice and appearance as a donkey are particularly unappealing, Titania's love for him seems to have changed his faults into virtues (in her mind anyway). We remember that Helena said pretty much the same thing about Demetrius's character flaws back in Quote #4.
Act 3, Scene 2
DEMETRIUS Lysander, keep thy Hermia. I will none. If e'er I loved her, all that love is gone. My heart to her but as guest-wise sojourned, And now to Helen is it home returned, There to remain. (3.2.72-76)
Demetrius has been enchanted, which should excuse him, but remember that this will be his second transformation in love. First he loved Helena, then he loved Hermia, and now he loves Helena again. Though we know this last transformation was caused by magic, he was fickle even before being enchanted.
HERMIA Why are you grown so rude? What change is this, Sweet love?
In this play, female characters have a harder time accepting the transformation of their loved ones. Here Hermia refuses to believe Lysander does not lover her, and earlier Helena had difficulty comprehending Demetrius's change of affections.