The most immediate cause for Brutus' foul temper in his confrontation with Cassius in Act IV, scene 3, is personal. His wife Portia, despairing of the success of her husband's cause, has committed suicide by the unpleasant method of swallowing fire. Brutus waits until the quarrel is over before telling this news to Cassius, who in astonishment exclaims,
How 'scaped I killing, when I cross'd you so?--
O insupportable and touching loss!--
Nevertheless, the details of the quarrel with Cassius reveal other motivations for Brutus to be ill-tempered. As Mark Antony will remark in his eulogy for the dead Brutus at the end of the play, Brutus is an idealist. Just as he shrunk from killing Mark Antony with Caesar, which would have been practical but immoral, he shrinks from the moral compromises that might be justified in a military emergency. One of Cassius' friends has been condemned for bribery; Cassius argues
In such a time as this it is not meet
That every nice offense should bear his comment.
but Brutus accuses him of the same crime, and nobly proclaims,
...shall we now
Contaminate our fingers with base bribes
And sell the mighty space of our large honours
For so much trash as may be grasped thus?
I had rather be a dog, and bay the moon,
Than such a Roman.
Brutus' ill temper is thus motivated not only by grief and tension but also by the poor fit between his native honest character and the political maneuvers he has gotten himself involved in.