why did portia commit suicide

Brutus has two different reactions to his wife's suicide, and I believe that like anyone else he kept himself private for the masses and opened up to his friends.

With Cassius, he answers his friend, and he shares his distress. Portia didn't want Caesar dead; she's a friend to Calpurnia and feels like she's an accessory because she's overheard Brutus and the other men planning. With the rest of the men, Brutus seems uncaring about her suicide.