Orsino sees Cesario, as a young squire just starting out in the world, much like himself as a young, spry lad, so he has a tendency to be more willing to unload onto her with his troubles and sorrows, seeking a companion with which to share and to teach. Thus, Viola grows in her male disguise to get a better feeling for his inner self, not the self that he shows to the public, or would reveal and share with Viola in her true female self, but rather his secret self, as he believes he shares with a peer. So, she grows to love him. But, Orsino's motivation is actually not love for Viola, but rather he seems to be in love with love itself. His entire world is filled with love but he knows that there might be a turning point for him, like when he says:
If music be the food of love, play on; give me excess of it, that, surfeiting, the appetite may sicken, and so die. 1. (I,I,I-III)
This quote shows that he knows that he is so caught up in "love", that he hopes his appetite for love may simmer when he takes more than he can handle.
1. Shakespeare, William. Twelfth Night. Longman's Canada Limited, Don Mills, Ontario, 1961. All subsequent quotes are from this edition....