This opening speech by the Duke immediately introduces the theme of love, the central one of the play. It explores the nature and effects of love, which appear to be overwhelming.
The Duke asks for music - ‘the food of love’, wanting to be overfilled with it so that his appetite for love might weaken:
If music be the food of love, play on,
Give me excess of it, that, surfeiting,
The appetite may sicken and so die. (I.i.1-3)
The imagery used here thus conveys his love as a physical appetite, linking it to the theme of sickness. He goes on to admit that the music after all is not enough to quench his love; although it is 'sweet' as the smell of violets (5-7) it cannot kill his desires.
Orsino now moves to directly address the ‘spirit of love’, which is always ‘quick and fresh’ (9) and limitless as the sea – although it can rise and fall like the tide (10-14). He ends this opening speech by a reference to ‘fancy’ (14) which seems to indicate that he is at least partly aware that his romantic musings are more to do with imaginings than reality. He has not actually begun any relationship with Olivia, the object of his desires, and indeed, has not had any physical contact with her at all.
The Duke’s opening speech, then, muses on the many-sidedness of love, commenting on both its emotional and physical impact, its overwhelming impact on all the senses as well as the soul. He also appears to idealise it for its own sake. It has often been noted that he is in love with love rather than with any actual person; later in the play he will discard Olivia quite quickly and take up with Viola.
In this opening speech the Duke appears literally lovesick; he has the time and leisure to indulge his romantic yearnings, and the languid motion and romantic imagery of his speech also reflect this. Both the positive and negative aspects of love that he articulates are later explored in the play. Love can exalt and overwhelm, but it is also linked with sickness and even madness.