What does the reader learn about Brutus from his reaction to Portia's death?Shakespeare's Julius Caesar

Interestingly, there seems to be the proverbial game of
cat-and-mouse in Scene 3 of Act IV when Titinius and Messala enter the tent of Brutus,
who tells Messala that he has received news that Octavius and Marc Antony are bearing
down upon them with a mighty army.  Messala concurs, and he adds that Antony has put to
death a hundred senators.  Messala then interjects a quiet question, "Had you your
letters from you wife, my lord?" Brutus tells him that he has not received anything even
though he really has, and he demands to know what news Messala bears.  Then, Messala
gives Brutus the news that Portia has eaten burning coals and killed herself. 
Stoically, Brutus replies,

Why, farewell, Portia.
We must die, Messala.
With meditating that she must die once
I
have the patience to endure it
now.(4.3.212-215)

Unbeknownst to Messala, Brutus
has already told Cassius that Portia is dead.  And, when Cassius exclaims, Brutus
stoically tells him,

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Speak no more of her. Give me a bowl of wine.

In this I bury all unkindness, Cassius.
(4.3.177-178)

Knowing that
others watch him and that Messala is aware of the dissension between himself and
Cassius, Brutus may feel that he needs to steel himself against displaying emotion. 
Instead, he encourages Cassius to act, telling him,  

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There is a tide in the affairs of men

Which taken at the flood leads on to fortune;
Omitted, all the
voyage of their life
Is bound in shallows and in miseries.
On such
a full sea are we now afloat,
And we must take the current when it serves,

Or lose our ventures.(4.3.244-250)

This fatalistic observation
by Brutus is also somewhat out of character for him, as well, further indicating that he
is focused upon the forthcoming battle.