If music be the food of love, play on.
This is the first line of the play, wherein Shakespeare establishes Orsino as extremely lovesick. When delivering this line, Orsino is surrounded by a band of performers who he is asking to play a certain song repeatedly in an attempt to make himself grow tired of it, and, as a result, fall out of love. He is hopelessly in love with Olivia, a woman who has no interest in him. As such, he hopes that, music being the "food of love" (which is to say, music has the power to make you fall more and more in love), then when he is exposed to the same song so many times, it will lose its appeal — just like love.
Some are born great, some achieve greatness, and some have greatness thrust upon them.
This line is spoken by Malvolio, who is reading a love letter that he believes to have come from Olivia. In actuality, the letter was written and planted for him to find by Maria, Fabian, Sir Toby Belch, and Sir Andrew as a prank. This line, "Some are born great, some achieve greatness, and some have greatness thrust upon them," is in reference to Malvolio discarding any uncertainties or hesitancies he may have regarding raising in rank upon becoming romantically involved with Olivia. The letter encourages him to dress and act a certain way in order to gain Olivia's favor, but in reality, it will only serve to make her uncomfortable and convinced that he is mad.
And what should I do in Illyria? / My brother he is in Elysium.
Viola speaks this line following the shipwreck that she and her brother, Sebastian, are separated in. She asks what country she is, to which she gets the response "Illyria." She wonders aloud what good being in Illyria does her, when her brother is most likely in Elysium — the afterlife for heroes in ancient Greek myth. This quote tells us two things about Viola and Sebastian's relationship: they are close (Viola does not know what she is supposed to do without him), and Viola regards Sebastian as a heroic, noble figure worthy of the Greek afterlife meant solely for heroes. With these two things in mind, Viola's devastation is understandable.
How have you made a division of yourself? / An apple cleft in two is not more twin / Than these two creatures. Which is Sebastian?
This line is uttered by Sebastian's friend, Antonio, after the twins are finally reunited. Neither of them assumed that the other had survived the shipwreck, so everyone is confused as to how there can suddenly be two Sebastians. Orsino comments on how they share the same voice and appearance, and how he cannot differentiate between Sebastian and Cesario (Viola, disguised as a man). With this reunion, the romantic conflicts of the play begin to rectify themselves, with the exception being Malvolio.
