One major theme found in Act 1, Scene 5 is the irrationality, or even madness of love. In this scene, Viola disguised as Cesario is sent by Duke Orsino to deliver his message of courtship to Olivia. It is during Olivia's and Viola's conversation that we learn Olivia really has no rational reason for rejecting Orsino's love. She even praises Orsino to be a good and noble person with unblemished character, as we see in her lines, "Yet I suppose him virtuous, know him noble, / Of great estate, of fresh and stainless youth" (I.v.243). She even carries on further to describe him as well-spoken, educated, brave, handsome, and even gracious, yet still insists she "cannot love him" (245-47). Since he has so many merits, one can easily say that her rejection of him is irrational. We can especially call it irrational when we know that her rejection primarily has to do with the fact that she is mourning her brother and father who both died about a year ago, and in her state of mourning, she wants nothing to do with any men.
Another way in which she behaves irrationally, even madly, is by falling in love with Cesario, even though he rebukes her with such terrible names as prideful, hardhearted, and even the "cruell'st she alive" (225). But she seems to fall for him because she finds him to be mysterious. First he was mysterious by being so persistent at seeing her, and even his looks are mysterious. Since Cesario is of course a woman, as Malvolio points out, she looks too young and underdeveloped to be a man and too old to be a boy. Yet for some irrational reason, Olivia seems to have fallen in love with his looks.