"If music be the food of love...."
With this oft-quoted speech, Shakespeare introduces not only the character of Duke Orsino but the character of the play as well. The speech is at once playful and melancholy, with abrupt changes of mood and a fickle, if languorous, quality to the language. The play, while reveling in its ludicrous nature, has its own changes of mood throughout the story—most notably the subplot involving the fall of Malvolio. But here it delights in letting Orsino romantically wax poetic, mooning over the unapproachable Olivia.
It's often said that Orsino isn't so much in love with Olivia as he is with the idea of being in love. Note that although ostensibly it's his fascination with Olivia that sets him in this mood, there is no specific mention of her in this speech. Indeed, as the scene unfolds, it is readily apparent that Orsino doesn't actually know Olivia. He has seen her, he has fallen for her, but his contact has been limited to the couriers he has sent who deliver his messages and plead his case. Then again, it is exactly this romantic quality in Orsino that makes us believe that he can fall in love with Viola when her guise is dropped.
In fact, Orsino is remarkably passive for someone in the throes of passion. Compare his pursuit of Olivia to most other relationships in the play. It only takes one conversation for Olivia to start mooning over Cesario (Viola). She sends her ring via Malvolio and then strings along Orsino so he'll keep sending Cesario over to speak with her. When Sebastian happens along, Olivia proposes to him (thinking him Cesario). Malvolio, once he's been set up, directly makes a pass at Olivia. Antonio risks his life and freedom for accompanying Sebastian to Illyria. Sir Toby marries Maria because of her part in tricking Malvolio. Orsino, in contrast, doesn't bother attempting a personal visit until the last scene of the play.
Whether either Orsino or Olivia are worth the effort is debatable. The duke is described by Olivia as noble, learned, valiant and gracious, among other things. Olivia is beautiful and devoted. Both characters, however, also demonstrate markedly self-absorbed—bordering on narcissistic—traits throughout the story. They both sequester themselves within their respective residences. They both aspire to an idealized love. And they both switch their love at the drop of a hat (or frock, if you prefer) when Viola's identity is revealed. In the balance, however, both matches seem for the best—although we have to extrapolate much of Sebastian's nature from his relation to Viola.
Of course, the play on the whole doesn't concern itself overmuch with the petty details of love. Rather, it's a play that dwells on the "high fantastical" nature of fancy, as Orsino puts it. Love may indeed come in many guises, but there's a delight in every form it takes.
If music be the food of love, play on;
Give me excess of it, that, surfeiting,
The appetite may sicken, and so die.
That strain again! It had a dying fall;
O, it came o'er my ear like the sweet sound,
That breathes upon a bank of violets,
Stealing and giving odour! Enough; no more:
'Tis not so sweet now as it was before.
O spirit of love! How quick and fresh art thou,
That, notwithstanding thy capacity
Receiveth as the sea, nought enters there,
Of what validity and pitch soe'er,
But falls into abatement and low price,
Even in a minute: so full of shapes is fancy
That it alone is high fantastical.
—Act I, sc. i