In Twelfth Night, why Orsino used Valentine & then Viola to convey his love to Olivia instead of courting her in person?

Illyria isn't called illyria for
nothing.  The people in this country all seem to be suffering from some kind of
love-sickness and they indulge themselves in this sickness.  None of them are being
honest to themselves let alone to anyone else.

Orsino
thinks he is in love with Olivia, perhaps because she is
unattainable since she is in love with the idea of being a martyr for her dead brother
and father.  Other characters like Toby, Sir Andrew, and Malevolio are likewise clueless
as to what love is.

It takes a castaway Viola disguised as
Ceasario to teach Orsino what love truly is.  Olivia falls in love with an image,
Viola/Caesario .  When she meets Sebastian, she has already fallen in love with him
physically.

There are a couple of reasons Orsino does not
go directly to Olivia but uses an emassary, first Valentine and then Caesario, is that
in that world a man did not directly court a woman but went through channels, so to
speak.  Secondly he respects the fact that she is in
mourning.

These are two practical reason.  Of course, from
a story telling point of view, it is necessary for Olivia to fall in love with Caesario
who as Viola is in love with Orsino.