By Act IV, Scene 2, their relationship has become a friendship, and it has become a friendship that has the decided qualities of a love relationship. In Act IV, Scene 2, Brutus has taken offense at what he believes was Cassius' refusal to send money when he needed it. Cassius is quite taken aback by this accusation and the conversation quickly descends into a "yes you did, no I didn't" affair that almost results in a fight. Cassius is innocent of the offense and is hurt that he is "Hated by one he loves, braved by his brother."
What motivates Brutus to this anger? It turns out that it is grief over Portia's death. It is to Cassius that Brutus turns in his grief. The grief that he feels, the loss, the sense of betrayal are all translated into anger toward this friend, and after those emotions are spent, the two men are closer in some ways than Brutus ever was with Portia. The latter relationship shares the same respect for each other and the same sharing of intimacy, yet it is a relationship that can operate in the same spheres because it encompasses a level of equality not possible between a woman and a man.
From that moment, the audience has an increasing amount of sympathy for Brutus, who has been humanized by his wife's death. While he clearly loved his wife, there was also some distance between them, partly because of her rather stoic nature (remember her self-wounding), partly because he is unwilling to confide in her. This combination of the masculine and the feminine in her character was not a completely appropriate one. It was unworkable given the way in which the Roman world worked. The flip side, of course, was Caesar's behavior. His combination of femininity and masculinity was also unworkable. With their deaths, Brutus is able to incorporate both aspects of their personalities, most directly from his wife, given her more moral nature. With the banishment of women and inappropriate femininity from Rome, the state ought to be a better one. But there is an unattractive sterility to such a world. What has been created is an unworkable ideal. Brutus' death is an indication of just how unworkable it is.
