How do the fairies in Shakespeare's A Midsummer Night's Dream affect the outcome of the play?

One of Shakespeare's most important themes in A
Midsummer Night's Dream is the irrationality or foolishness of mankind.
Shakespeare portrays the foolishness of mankind by pointing out that love is an
irrational emotion. Demetrius is especially one character he uses to prove the
irrationality of love and mankind in general. In addition, Shakespeare uses the
fairies to both point out the foolishness of mankind and to
resolve the characters' foolishness. The
fairies affect the outcome of the play by pairing the
couples in a way that is sensible and rational and also by blessing their
marriages.

The irrationality of love is first pointed out by Helena in
the opening scene. Helena is bemoaning the fact that Demetrius has abandoned loving her
for Hermia, even though he has no rational reason for it. For instance, Helena points
out that throughout all of Athens she is thought to be just as beautiful as Hermia, as
we see in her line, "Through Athens I am thought as fair as she" (I.i.232). Hence,
Helena is pointing out that Demetrius's reasons for ceasing to love her are not based on
objective reality, but rather his subjective opinion. Helena further points out that
love is not based on anything objective and real when she declares, "Love looks not with
the eyes, but with the mind; / And therefore is wing'd Cupid painted blind" (239-240).
Because love is based solely on the irrational imagination, we see just how foolish love
truly is and how foolish mankind is as well. Even Puck
illustrates the foolishness of mankind in his famous line, "Lord, what fools these
mortals be!" (III.ii.116).

Since it is perfectly foolish and
irrational for Demetrius to be pursuing Hermia instead of Helena, especially when Hermia
is in love with Lysander, the fairies amend this foolish
situation by rightly pairing the couples. Puck goofs things up at first, adding more
conflict and creating the climax; but, based on Oberon's commands, he eventually makes
Demetrius fall in love with Helena and makes Lysander fall back in love with Hermia.
Pairing the lovers this way is sensible and rational because now neither woman is alone
and abandoned with a broken heart, and as Puck phrases it, each "Jack shall have [a]
Jill" (III.ii.481). Demetrius even points out the rationality of the match when he
confesses to Theseus that he was engaged to Helena before he saw Hermia, showing us that
renewing his faithfulness to Helena is really the only rational way to behave. We see
Demetrius's confession to their engagement in his line, "To her, my lord, / Was I
betroth'd ere I saw Hermia" (IV.172-173).

Not only do the fairies
affect the outcome of the play by appropriately pairing the lovers, they further affect
the outcome by blessing the lovers in the end. One of the purposes of their blessing is
to ensure that the lovers will remain in love and forever true to each other, as we see
in Oberon's line, "So shall all the couples three / Ever true in loving be"
(V.i.402-403).