In Twelfth Night, Shakespeare portrays love as something wild and passionate, something that takes people almost out of their senses and makes them do things they normally would not do, something that comes upon them suddenly and often makes them miserable. Yet he also depicts the triumph of love in the end (when the characters finally find the right person to love).
As the play begins, Duke Orsino laments his unrequited love for Olivia. He is miserable but determined. Olivia has renounced all love as she grieves for her brother, but the Duke will not take no for an answer. He bothers Olivia unendingly with proposals, vowing not to leave her alone until she says yes to him. He sends messenger after messenger, but finally he sends one too many, and Olivia, too, falls in love, just not with the Duke!
The last messenger Orsino sends to Olivia is Viola, disguised as the young man Cesario. Olivia falls head over heals for Viola/Cesario, but Viola, of course, is not at all interested, for she has fallen in love with the Duke. Viola manages to control herself (she must to maintain her disguise), but Olivia does not. She chases “Cesario” shamelessly, behaving in a way that would normally be completely incompatible with her serious nature.
Somehow, though, Olivia fails to notice that Viola's brother, Sebastian, is not Cesario. Yes, the siblings look a lot alike, but the fact that Sebastian has no idea who she is or what she is talking about should have tipped her off that something is amiss. Olivia's love blinds her, however, and she gets a promise of marriage out of Sebastian and even a betrothal ceremony, for he has fallen in love with her as well, perhaps simply out of the fact that a beautiful woman is in love with him.
In the end, however, everything works out for the best. Cesario is unmasked as Viola, and the Duke decides he can love her just as well as he can love Olivia and maybe even better since she returns his love. Olivia and Sebastian remain deliriously happy, and love seems to have triumphed all around.