How does Shakespeare show love in Twelfth Night?
Two sides of love that Shakespeare depicts in Twelfth Night are unreasoned love at first sight and the elements of successful love. The first, love at first sight, is depicted in the characters of Orsino ("If this be the food of love, play on; /... to sweet beds of flowers; / Love-thoughts lie rich when canopied with bowers."), Viola, Olivia and Sebastian.
Orsino confesses that when he first saw Olivia, he instantly turned to loving her. Viola goes to work for Orsino disguised as Cesario and within three days she is in love with Orsino. Olivia hears Cesario/Viola speak one time and falls in love. Sebastian consents to be taken by a lady who is essentially a stranger to be united in marriage. These are unreasoned loves.
The Shakespearean theme that ties music to love gives a glimpse at those who metaphorically sing the song of love well enough to be successful and those who don't. The key to success lies with Viola. She says in Act I that she can sing to lots of kinds of music and therefore is sure to be able to please. Feste supports this in the final scene of the play when he sings that bragging and boasting never won in marriage suits.
Interestingly, Sir Andrew fails to impress Maria because his musical instrument isn't impressive enough. Shakespeare doesn't suggest that what is not good enough for person X will be good enough for person Q because it is all a matter of perception. On the contrary, he suggests that succeeding or losing in love depends on how well it is done by the suitor.