Throughout the play, Portia is devoted to her husband, and concerned about what might happen to him. She notices that he seems burdened, and, associating his behavior with the visits of Cassius and the other conspirators, wants to know what is going on. Her suicide, or at least Brutus's description of it to Cassius, suggests that she has invested her entire life in Brutus. In particular, she is saddened by the apparent rise to power of his rivals Octavius and Antony. Brutus says, in any case, that this is why she killed herself:

Impatient of my absence, 
And grief that young Octavius with Mark Antony
Have made themselves so strong: for with her death 
That tidings came: with this she fell distract, 
And, her attendants absent, swallow'd fire.

With Brutus gone, and likely to be killed by his rivals, Portia seems to have found life to be not worth living. She kills herself in what appears to be a moment of madness brought on by her circumstances.