How does the author introduce the theme of unrequited or unanswered love?

Orsino's love for Olivia is an example of unrequited love.

If music be the food of love, play on;

Give me excess of it, that surfeiting,

The apetite 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 odor. Enough, no more.

'Tis not so sweet now as it was before.

O spirit of love, how wquick and fresh art thou,

That, notwithstanding thy capacity

Receiveth as the sea, naught 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...

O, when mine eyes did see Olivia first,

Methought she purged the air of pestilence.

That instant was I turned into a hart,

And my desires, like fell and cruel hounds,

E'er since pursue me.