Twelfth Night is predominantly a satire of the ideas of love professed by and made popular by the Medieval poet Francesco Petrarch ("Shakespeare's Twelfth Night"). Petrarch wrote over 300 sonnets containing the subject of a woman named Laura. In these sonnets, he idealized the beautiful Laura, putting her on a pedestal, creating an unequal relationship between the lover and one who is loved. His sonnets also professed the necessity to suffer for the sake of the beloved, likening love to feelings of freezing and burning ("Juliet Trumps Laura: Shakespeare and the Petrarchan Sonnet"). Twelfth Night satirizes Petrarchan love by first having Duke Orsino idealize Olivia in Petrarchan fashion but then being taught to be more reasonable and practical when he decides to accept Viola as a wife instead of continuing to pine for Olivia.
We see signs of Orsino placing Olivia on a pedestal and suffering over his unrequited love for her all throughout the play, but especially in the very first scene. Here we see just how much he has idealized Olivia when he makes his metaphorical claim that he believes Olivia's beauty can heal plagues, as we see in his lines, "O, when mine eyes did see Olivia first, / Methought she purg'd the air of pestilence!" (I.i.20-21). In this same speech, we also see the extent to which he has allowed himself to suffer over the thought of her when he likens his heart to a "hart," or stag, being hunted by his desires, which he likens to cruel hunting dogs:
That instant was I turn'd into a hart;
And my desires, like fell and cruel hounds,
E'er since pursue me. (22-24)
However, Shakespeare does not allow him to remain in this frame of mind, showing us that Shakespeare is really satirizing these Petrarchan ideals. Throughout the play, we are given hints that Orsino has actually become disenchanted with pining over Olivia and is starting to find himself falling for who he thinks is his new manservant Cesario. We even see this intimated in Act 1, Scene 4, just before Orsino sends Cesario out to court Olivia for him. Orsino comments that Cesario looks nothing like a man and even has lips as red as Diana's and a maiden's throat (I.v.31-34). In addition, Viola as Cesario had earned Orsino's affection through her devout friendship and loyalty to him as she continued to do his bidding, despite the fact that she was in love with Orsino herself. Hence, by the time Orsino decides to make Viola his wife in the final scene, we see that Orsino has gained a more reasonable, rational understanding of love, which satirizes Patrarch's irrational interpretation of love.