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
Alas, their love may be call'd appetite,
No motion of the liver, but the palate,
That suffer surfeit, cloyment and revolt;
But mine is all as hungry as the sea,
And can digest as much: male no compare
Between that love a woman can bear me And that I
owe Olivia.

This is what Duke Orsino says to Cesario so he shows how much he loves and cares about Olivia.
He says that she deserves the best and that he is the one. Again this must be very painful for Cesario/Viola to hear because it is always hard to hear that the person you love is instead in love with someone else.