If music be the food of love, play on.
Give me excess of it; that surfeiting,
That appetite may sicken and die (I.i.)

Duke Orsino’s soliloquy forms the first lines spoken in the play. The speech introduces the importance that love will play in the plot. Orsino, as we soon learn, is in love with Lady Olivia. But Lady Olivia is not moved by the Duke’s advances, leaving Orsino in a rather uncomfortable position. He is burdened by lovesickness and wants relief. If music is the nourishment of love, he thinks, then perhaps by glutting himself on music, he can also become sick of love and so his desires might conveniently go away. In a sense, he’s eager to move on with his life. This soliloquy establishes one of the central themes of Twelfth Night: love as a powerful force with a will of its own.

What is love? ’Tis not hereafter;
Present mirth hath present laughter;
What’s to come is still unsure;
In delay there lies no plenty (II.iii.)

These lines are sung by Feste in Olivia’s house at the bequest of Sir Toby. The song emphasizes the limited shelf life of romantic love. Love is something that ought to be seized quickly. The song also strongly reflects the recent developments in Olivia’s storyline. After resolving to mourn her brother for seven years without showing her face in public, she has suddenly fallen head over heels for Ceasrio, aka Viola. Now Olivia seems to appreciate that “in delay there lies no plenty.” In order to follow her heart, she must seize the moment.

We men may say more, swear more, but indeed
Our shows are more than will, for still we prove
Much in our vows, but little in our love. (II.iv.)

Here Viola, disguised as Cesario, discusses with Orsino the different ways in which men and women love. After Orsino suggests that women cannot love as intensely as men do because their hearts cannot retain as much passion, Cesario tells him the story of a fictional, lovelorn sister in order to demonstrate how misguided Orsino’s prejudice is. At the end, Cesario suggests that even though men may express their love more emphatically than women, men will not necessarily be faithful to their partner or consistent in their affection. And indeed, Orsino’s sudden decision to marry Viola near the end of the play seems to support this observation.