How does the magical herb in Shakespeare's A Midsummer Night's Dream metaphorically represent infatuation?

In Act 2 scene 1 of A Midsummer Night's Dream, Oberon tells Puck how he watched Cupid shoot his arrow at a “fair vestal.” Cupid misses his target, but Oberon notes where the arrow fell:

Yet mark'd I where the bolt of Cupid fell:
It fell upon a little western flower,
Before milk-white, now purple with love's wound,
And maidens call it love-in-idleness.
Fetch me that flower; the herb I shew'd thee once:
The juice of it on sleeping eye-lids laid
Will make or man or woman madly dote
Upon the next live creature that it sees.

Puck indeed fetches the flower for Oberon, who means to get revenge on Titania with the flower’s magic. (Titania has refused to give him a favorite human boy she fosters.) Note that the juice of the purple flower applied to the eyelids of a sleeping man or woman will make them “madly dote” on the next creature he or she sees.  This involuntary mad doting is irrational and random.  As soon as the person wakes, he or she becomes infatuated with the first live being they lay eyes upon.

Maidens call the flower “love-in-idleness,” an appropriate title that suggests infatuation’s vacant and empty “love." We watch as Lysander, mistaken for scornful Demetrius because of his Athenian garb, is the first victim as Puck applies the flower’s magic juice to his eyelids. Once sincerely devoted to Hermia, Lysander wakes to see Helena and immediately becomes obssessed with her.  He forsakes and denies Hermia and follows Helena everywhere while pledging his love. 

Oberon then scolds Puck for applying the magic to the wrong Athenian. Puck amends this mistake by making sure that Demetrius gets an application of the magic juice.  Lysander and Demetrius both now follow Helena, swearing love to her and squabbling with each other foolishly.

Finally, Titania--also a recipient of the flower's magic--awakes to see Bottom with his donkey’s ears and is instantly in love with him, praising his singing and calling his beauty equal to his wisdom.

Titania’s “love” for Bottom is the epitome of the folly of infatuation. We can see how silly her infatuation is--even Bottom has doubts that she could truly love him. Titania continues to dote upon Bottom, however, and makes her fairies cater to his every whim.

When Oberon finally wakes Titania from her spell, Titania cries:

My Oberon! what visions have I seen!
Methought I was enamour'd of an ass...

O, how mine eyes do loathe his visage now!

Thus the magic flower shows how infatuation--defined by the OED online as “an extravagantly foolish and unreasoning passion”--is consuming, obsessive, irrational, and ridiculous. When the victims of the magic awake, they are sheepish about and embarrassed by their behavior, much as a real people might be as they come to their senses and see through their flimsy infatuations.

What might Oberon's magical herb metaphorically represent in A Midsummer Night's Dream?

The term "falling in love" itself implies Shakespeare's motif of imbalance in his romantic comedy, A Midsummer Night's Dream. Thus, the magical herb which Oberon in Act II, Scene 1 about which Oberon gives Puck orders,

Fetch me that flower; the herb I shew'd thee once:
The juice of it on sleeping eye-lids laid
Will make a man or woman madly dote
Upon the next live creature that it sees.

Certainly, this metaphor of love out of balance is exhibited by Romeo in another of Shakespeare's plays as, without any herb, the young, impulsive Romeo falls madly in love with the daughter of his mortal enemy, the fair Juliet. Thereafter they are "star-crossed lovers," out of balance with the universe.

Likewise, when Puck disseminates this flower's magical powers, it throws the young humans into an asymmetrical love as of the four, one young woman has two men in love with her while another woman has no lover. This imbalance must be set aright for harmony to be achieved; therefore, the play concludes with the symmetry of couples restored, and the madness of love's imbalance ends.