Every major character in Twelfth Night experiences some form of desire or love. Duke Orsino is in love with Olivia. Viola falls in love with Orsino, while disguised as his pageboy, Cesario. Olivia falls in love with Cesario. This love triangle is only resolved when Olivia falls in love with Viola's twin brother, Sebastian, and, at the last minute, Orsino decides that he actually loves Viola. Twelfth Night derives much of its comic force by satirizing these lovers. For instance, Shakespeare pokes fun at Orsino's flowery love poetry, making it clear that Orsino is more in love with being in love than with his supposed beloveds. At the same time, by showing the details of the intricate rules that govern how nobles engage in courtship, Shakespeare examines how characters play the "game" of love.
Twelfth Night further mocks the main characters' romantic ideas about love through the escapades of the servants. Malvolio's idiotic behavior, which he believes will win Olivia's heart, serves to underline Orsino's own only-slightly-less silly romantic ideas. Meanwhile, Sir Andrew Aguecheek, Sir Toby Belch, and Maria, are always cracking crass double entendres that make it clear that while the nobles may spout flowery poetry about romantic love, that love is at least partly motivated by desire and sex. Shakespeare further makes fun of romantic love by showing how the devotion that connects siblings (Viola and Sebastian) and servants to masters (Antonio to Sebastian and Maria to Olivia) actually prove more constant than any of the romantic bonds in the play.
During the Renaissance, melancholy was believed to be a sickness rather like modern depression, resulting from an imbalance in the fluids making up the human body. Melancholy was thought to arise from love: primarily narcissistic self-love or unrequited romantic love. Several characters in Twelfth Night suffer from some version of love-melancholy. Orsino exhibits many symptoms of the disease (including lethargy, inactivity, and interest in music and poetry). Dressed up as Cesario, Viola describes herself as dying of melancholy, because she is unable to act on her love for Orsino. Olivia also describes Malvolio as melancholy and blames it on his narcissism.
Through its emphasis on melancholy, Twelfth Night reveals the painfulness of love. At the same time, just as the play satirizes the way in which its more excessive characters act in proclaiming their love, it also satirizes some instances of melancholy and mourning that are exaggerated or insincere. For instance, while Viola seems to experience profound pain at her inability to be with Orsino, Orsino is cured of the intense lovesickness he experienced for Olivia as soon as he learns that Viola is available.
The theme of madness in Twelfth Night often overlaps the themes of desire and love. Orsino talks about the faculty of love producing multiple changing images of the beloved, similar to hallucinations. Olivia remarks at certain points that desire for Cesario is making her mad. These examples of madness are mostly metaphorical: madness becomes a way for characters to express the intensity of their romantic feelings.
But the play also has multiple characters that seem to go literally mad. As part of the prank that Maria, Sir Toby, and Fabian play on Malvolio, they convince everyone that he is crazy. The confusion that results from characters' mixing up Viola/Cesario and Sebastian, after Sebastian's arrival in Illyria, also leads many of them to think that they have lost their minds. The general comedy and chaos that creates (and results from) this confusion also references the ritualized chaos of the Twelfth Night holiday in Renaissance England.
Characters in Twelfth Night constantly disguise themselves or play parts in order to trick those around them. Some of the most notable examples of trickery and role-playing in Twelfth Night are: Viola disguising herself as the page-boy Cesario; Maria and Sir Toby playing their prank on Malvolio; and Feste dressing up as the scholar, Sir Topas. More subtly, Orsino's rather clichéd lovesickness for Olivia and Olivia's just-as-clichéd response as the unattainable mourning woman bring into question the extent to which these characters are just playing these roles, rather than truly feeling the emotions they claim to be experiencing.
Through the constant performance and role-playing of his characters, Shakespeare reminds us that we, like the characters, may play roles in our own lives and be susceptible to the role playing of others.
In connection with the themes of deception, disguise, and performance, Twelfth Night raises questions about the nature of gender and sexual identity. That Viola has disguised herself as a man, and that her disguise fools Olivia into falling in love with her, is genuinely funny. On a more serious note, however, Viola's transformation into Cesario, and Olivia's impossible love for him/her, also imply that, maybe, distinctions between male/female and heterosexual/homosexual are not as absolutely firm as you might think.
The play stresses the potential ambiguity of gender: there are many instances in which characters refer to Cesario as an effeminate man. Even more radically than this, however, it also suggests that gender is something you can influence, based on how you act, rather than something that you are, based on the sexual organs you were born with. Twelfth Night also shows how gender-switches make the characters' sexual identities unstable. For instance, at times, Olivia seems to be attracted to Cesario because "he" is such a womanly-looking man, while Orsino at the end of the play seems as attracted to Cesario as he is to Viola.
In Twelfth Night, as in many Shakespearean comedies, there are many similarities between a "high" set of characters, the masters or nobles, and a "low" set of characters, the servants. These separate sets of characters and their parallel plots provide comic counterpoint and also reflect the nature of the Twelfth Night holiday, which was typically celebrated by inverting the ordinary social order—a commoner or fool would dress up and get to play the king. The clown Feste's constant mocking of his "betters" further reinforces this idea of upsetting the social order.
Class and social standing is also a recurring theme in Twelfth Night. The priggish Malvolio is obsessed with status, always condescending to the other servants for their lowliness and dreaming of marrying Olivia and becoming a Count. Sir Andrew Aguecheek also wants to marry Olivia, but stands no chance because of his vulgarity and crassness. In marrying Olivia, even the noble Sebastian is moved in part by her wealth and social standing. Viola, at the beginning of the play, has lost her wealth in a shipwreck and in disguising herself as a page-boy is impersonating a different class from her own. Viola's disguise suggests that class, like gender identity, is to some extent a changeable role that you play by adopting a certain set of clothing and behaviors.