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:
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.
I'm not sure Twelfth Night has a "moral," but it does contain several important lesson. The first one is that things are not always what they seem. This is particularly exemplified by Viola. She disguises herself as a boy, fooling both her employer Orsino, and her employer's love Olivia. She is not what she is, as she tells Olivia. So, maybe the moral is "Pay attention!" Other characters who are not what they seem to be abound in the play.
There is Feste, who seems to be a fool, but who is actually a keen observer of men who really are foolish, among them Toby and Sir Andrew, both of whom seem to be knights of the realm, but are really fools.
Maria seems to be a mere servant, yet she is clearly in a position of authority with the lady whom she serves.
Olivia seems to be a young woman in mourning for her brother and her father, but in reality, she is someone who is capable of running an entire estate.
Orsino appears to be a noble duke, but he is really a petulant, lovesick young man.
The only person who really is what he is is Sebastian, and he is the one who causes the house of cards to fall down in the end.