After Feste’s exit, Orsino decides that he’s had enough of everyone, dismisses everyone but the main nuncio of the theme–
Orsino: Let all the rest give place: Once more, Cesario,
Get thee to yond same sovereign cruelty:
Tell her my love more noble than the world
Prizes not quantity of dirty lands–
The parts that fortune hath bestow’d upon her:
Tell her I hold as giddily as Fortune–
But ’tis that miracle, and Queen of Gems
That nature ‘dorns her in, attracts my soul.
Orsino, having caught onto the money-blunder fault with Feste earlier, basically instructs Cesario to get back to Olivia to tell her that he doesn’t want her for her money (“prizes not quantity of dirty lands”), and in fact, he views those “parts that fortune hath bestow’d upon her… as giddily as Fortune” (in other words, he’s too rich to care about more money). Rather, Orsino’s soul loves her for the miracle of her beauty, “adorned her by nature”.
Viola, fearing the worst, asks:
Viola: But, if she cannot love you sir.
Orsino can’t take that:
Orsino: It cannot be so answer’d.
Viola, completing Orsino’s pentameter, tries to reason with him, so what if some lady loves you with as much love as you do Olivia, and you cannot love her. She’d have to take that:
Sooth but you must.
Say that some Lady, as perhaps there is,
Hath for your love as great a pang of heart
As you have for Olivia: you cannot love her –
You tell her so — Must she not then be answer’d?
Orsino then gives a rather misogynistic worldview, of no woman’s body being able to withstand the “beating of so strong a passion / as love doth give [his] heart.” Moreover, he claims a woman’s heart can’t be “so big, to hold so much, [because] they lack retention.” It’s curious how Orsino opened the play wishing to have his love quenched by a gluttony of it, and now his view on love has it such that his is “as hungry as the Sea, / and can digest as much.” Basically, “mak no compare / between that love a woman can bear me.”
There is no woman’s sides.
Can bide the beating of so strong a passion,
As love doth give my heart: no woman’s heart
So big, to hold so much, they lack retention.
Alas, their love may be call’d appetite,
No motion of the Liver, but the Pallate,
That suffer surfeit, cloyment, and revolt,
But mine is all as hungry as the Sea,
And can digest as much, make no compare
Between that love a woman can bear me,
And that I owe Olivia.
Viola replies immediately, with melancholy:
Viola: Aye, but I know.
Orsino is surprised at Viola’s quick response, pauses, perhaps cocks an eyebrow, before curiosity takes over at this young page’s impertinence:
Orsino: What dost thou know?
Viola admits she knows “too well,” the kind of love women have for men, and that this love is “as true of heart” as “ours (men’s).” She then goes on to mention her father’s daughter, who’d loved a man, the same way that Cesario might love his Lord–were Cesario a woman, that is!
Too well that love women to men may owe:
In faith they are as true of heart, as we.
My Father had a daughter lov’d a man
As it might be perhaps, were I a woman
I should your Lordship.
Intrigued, Orsino immediately asks for her story:
Orsino:   And what’s her history?
Viola begins her tale with the classic result of repressed unrequited love, “Nothing happened, because she never told him… She pined away, patiently like a statue, but wasted away, smiling at this bittersweet unrequitedness.” Viola challenges Orsino’s bulimic appetite with this silent death of a love (that’s very real and present to herself), asking if this is love, and concluding that men say a lot, but it’s all “a show of vows,” as the love isn’t really sinecere:
A blank, my Lord: she never told her love,
But let concealment like a worm i’th bud
Feed on her damask cheek: she pin’d in thought,
And with a green and yellow melancholy,
She sat like Patience on a Monument,
Smiling at grief. Was not this love indeed?
We men may say more, swear more, but indeed
Our shows are more than will: for still we prove
Much in our vows, but little in our love.
Enchanted by this story of a woman with love so strong, that she would waste away, Orsino asks the blunt question–did she actually die:
Orsino: But died thy sister of her love, my Boy?
Viola, also enchanted by her own story, answers too close to the truth, referring herself to herself as her father’s daughter, and all the brothers as well, but she knows not. Then, she makes a quick exit by changing the topic back “on theme”:
I am all the daughters of my Father’s house,
And all the brothers too; and yet I know not.
Sir, shall I to this Lady?
Orsino, realizing that they’d greatly digressed from his initial intent, gets back on topic. It seems as if he’s enacting the proof of how men’s vows are more show than anything else, as he gives Cesario a jewel to give Olivia (even though wealth is not the reason for his courting):
Aye, that’s the Theme,
To her in haste: give her this Jewell: say,
My love can give no place, bid no denay.