One of the dominant love themes in Twelfth Night is the arbitrariness or foolishness of love. We especially see both the arbitrariness and foolishness of love with respect to both Duke Orsino and Oliva. Duke Orsino proves to be foolish with respect to his persistent obsession over Olivia. He is so obsessed with her that he relentlessly pines for her, visualizing himself as a hart, or deer, being hunted by his emotions. Even the fact that she rejects him due to being in excessive mourning over her brother's death makes him pine for her even more. His line of reasoning is that if she can love a brother so deeply, how much more would she love a man she was in love with?
Olivia portrays the theme of both the arbitrariness and foolishness of love first through her rejection of Orsino and then through falling for Viola as Cesario. Olivia really does not have a rational reason for rejecting Orsino, especially considering she admits she knows him to have an exceptional, noble character, as we see in her lines:
Your lord does know my mind; I cannot love him
Yet I suppose him virtuous, know him noble,
Of great estate, of fresh and stainless youth;
In voices well divulged, free learn'd, and valiant. (I.v.242-245)
These lines actually describe Orsino as quite a great catch; therefore, she is being portrayed as foolish for rejecting him. Not only that, she falls for Cesario who, based on Malvolio's descriptions of her, looks absolutely nothing like a real man, but more like a cross between a boy and a man. Hence, she has arbitrary reasons for preferring Cesario, showing us again the theme that love is both foolish and arbitrary.