Orsino's opening speech, his love for Olivia, and the news that she is mourning and will see no one.
Duke Orsino is in his palace, surrounded by lords and musicians. Music is playing. He opens with one of the most famous lines in Shakespeare: "If music be the food of love, play on; / Give me excess of it, that surfeiting, / The appetite may sicken and so die." He wants to be saturated with music until desire burns itself out. Within the same breath he tells the musicians to stop: the sound has already lost its sweetness.
A lord named Curio suggests they go hunting. Orsino picks up the word and turns it on himself: he is the hart, and his desires are the hounds pursuing him since the day he first saw Olivia.
Valentine, a gentleman, returns from Olivia's house with her refusal. Olivia has vowed to mourn her dead brother for seven years: she will wear a veil, refuse all visitors, and weep daily to keep his memory fresh. Orsino hears this not as rejection but as proof of her capacity for love. If she can feel so strongly for a dead brother, he reasons, she will love him even more powerfully once Cupid's arrow strikes.
The opening speech is a performance. Orsino does not simply feel love; he stages it. He calls for music, describes his desire in elaborate sensory language ("it came o'er my ear like the sweet sound / That breathes upon a bank of violets"), and asks to be overwhelmed so completely that love will destroy itself. The ambition is theatrical: he wants the maximum possible sensation, then wants it to stop. Within a few lines, both happen. He is satisfied and dissatisfied almost simultaneously, which Feste will later describe more accurately than Orsino ever does himself, calling his mind as changeable as an opal.
The hart / hounds metaphor is deliberately ironic. A hart is both a male deer and the homophone of "heart." Orsino tells Curio they will hunt the hart, then pivots: he is the hart, his own desires pursuing him. He has taken a scene of masculine action and collapsed it inward into an image of being hunted by his own feelings. The metaphor sets up a pattern the whole play will develop: Orsino is more absorbed in his own romantic suffering than in any real action.
Valentine's news about Olivia does not deflate Orsino at all. He interprets her seven-year vow of mourning as a sign of extraordinary love and immediately translates it into a promise about himself: a heart capable of such devotion to a brother will, once directed at a husband, be twice as powerful. This is almost delusional. He has taken Olivia's rejection as proof of her suitability for him. The gap between what he imagines and what is actually happening will define his character for the rest of the play.