What is the moral of Twelfth Night?

As a romantic comedy, Twelfth Night
is about love, and it certainly teaches the audience some lessons about love. While
love in this play love is true, but it is also fickle, irrational, and excessive.  Love
wanes over time, as does its chief cause, physical beauty.  As the play opens, Duke
Orsino expresses the idea of love's excess and
waning,

If
music be the food of love, play on,

Give me excess of it,
that, surfeiting,

The appetitie may sicken, and so
die.

That strain again! It had a dying fall;
....

Enough!  No more!
(1.1.1-11) 

Orsino also
demonstrates the irrationality and fickleness of love as he pursues Olivia recklessly,
but at the end of the play, he gives her up to Sebastian, then falling in love with
Cesario when "he" reveals himself to be Viola.

Moreover,
love is also madness.  In Act I, after seeing Cesario for the first time, Olivia is
madly in love:

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I do I know not what, and fear to
find

Mine eye too great a flatterer for my
mind.

Fate, show thy force; ourselves e do not
owe.

What is decreed must be--and be this so!
(1.5.309-312)

The denial of
Sebastian that his love for Olivia is madness certainly points to the connection between
unbounded passion and madness. For instance, in Act IV, Sebastian says that he is
willing to "distrust mine eyes" because of his love for Olivia.  In addition, love
is mad because it is connected to witchcraft and being possessed by the devil. Of
course, the best example of the foolishness and madness of love is in the character of
Malvolio, whose professions of love for Olivia lead to his being restrained as a
lunatic. 

In Shakespeare's Twelfth Night, Love is "a many
splendored thing," indeed.  It is fickle, excessive, irrational, and
mad.