No one is supposed to know what is bothering Brutus, and you could argue that people learning the cause of his worries is a driving force in the play: it is only when Cassius discovers Brutus's thoughts about Caesar that he pushes Brutus to take action and the conspiracy is...
No one is supposed to know what is bothering Brutus, and you could argue that people learning the cause of his worries is a driving force in the play: it is only when Cassius discovers Brutus's thoughts about Caesar that he pushes Brutus to take action and the conspiracy is formed. Brutus excludes Portia from his worries because he is afraid of drawing her into whatever the fallout of his actions may be.
Brutus is a quiet sort of person, and has heretofore kept his concerns about Caesar to himself until he first speaks to Cassius and Casca in act I. The conspiracy they form troubles Brutus greatly, for Caesar has been a friend and father figure to him, but he comes to believe it is necessary to remove Caesar from power for the good of the Roman Republic. The question of when and how to effect this removal weighs heavily on Brutus's mind. He is moody and preoccupied, as Cassius notes:
Brutus, I do observe you now of late:
I have not from your eyes that gentleness
And show of love as I was wont to have:
You bear too stubborn and too strange a hand
Over your friend that loves you.
Brutus acknowledges how he must seem and explains:
Be not deceived: if I have veil'd my look,
I turn the trouble of my countenance
Merely upon myself. Vexed I am
Of late with passions of some difference,
Conceptions only proper to myself[.]
Cassius is able to draw out Brutus's thoughts and direct them toward some action by forming a conspiracy with Brutus to oust Caesar. Assassination is not mentioned at this point, but Brutus realizes later that killing Caesar is the only way to remove him from power. He cannot quite bring himself to accept this and is debating with himself in his orchard that evening:
It must be by his death: and for my part,
I know no personal cause to spurn at him,
But for the general.
Since Cassius first did whet me against Caesar,
I have not slept.
Between the acting of a dreadful thing
And the first motion, all the interim is
Like a phantasma, or a hideous dream[.]
Into this context, Brutus's wife, Portia, enters. She loves her husband, and she is no fool; she knows something is troubling him deeply. She also knows that several men have just been at the house in the middle of the night, so something is obviously afoot. She begs to be allowed into his confidence; she can see that whatever is bothering him is making him ill with worry:
It will not let you eat, nor talk, nor sleep,
And could it work so much upon your shape
As it hath much prevail'd on your condition,
I should not know you, Brutus. Dear my lord,
Make me acquainted with your cause of grief.
Brutus tries to put her off, claiming he's simply unwell, but Portia prevails on him for the truth, saying,
You have some sick offence within your mind,
Which, by the right and virtue of my place,
I ought to know of[.]
She drops to her knees to plead with him for answers, swearing that whatever the issue is, she will keep his secrets faithfully, even as she has kept secret a stab wound she gave to herself, giving him no indication of its pain, to prove to him how well she can keep his confidence:
Tell me your counsels, I will not disclose 'em:
I have made strong proof of my constancy,
Giving myself a voluntary wound
Here, in the thigh: can I bear that with patience.
And not my husband's secrets?
As a woman, it's arguable that Portia is not "supposed" to know what's on Brutus's mind in case she foolishly betrays him. It was commonly believed that women were not capable of keeping secrets, and Portia does struggle:
O constancy, be strong upon my side,
Set a huge mountain 'tween my heart and tongue!
I have a man's mind, but a woman's might.
How hard it is for women to keep counsel!
She masters her "weak" character, however, out of love for her husband, and at this, Brutus relents and agrees to let her in on the truth. She has proven (in a rather grotesque and alarming way) that she is capable of keeping secrets, and in so doing she has proven her love for Brutus. He discloses everything to her, so that the next morning—the fateful Ides of March—Portia waits anxiously for news from the Senate, knowing what Brutus has planned. She takes no direct action but is a kind of accessory to the crime, a co-conspirator with her husband who, through her silence, ensures Brutus's success.