We learn in Act IV of Portia's death when Brutus gives this news to Cassius after they have argued violently. Before telling Cassius of Portia's suicide, Brutus says that he is "sick of many griefs" and that "[n]o man ears sorrow better [than he]." After sharing the details of Portia's despair and suicide, Brutus tells Cassius to discuss Portia no further. A short time later, when Cassius is still trying to accept that Portia is dead, Brutus says to his friend, "No more, I pray you." Brutus then turns to the business at hand.
We can infer from Brutus' reactions in this scene that he cannot bear to discuss his wife or to dwell upon her death--and the nature of her death--because doing so brings him such great pain. "I pray you" suggests that he most earnestly needs Cassius to move on to another subject. Cassius understands the grief Brutus experiences, calling Portia's death an "insupportable and touching loss."