Why do cassius and brutus take their lives at the end of the tragedy of julius caesar?

Assius and Brutus take their own lives at the end of Julius Caesar to avoid being captured by the enemy. Cassius knows he will soon be captured by Antony and Octavius. He will more than likely be dragged through the streets of Rome in chains so he orders Pindarus to hold his sword ready and he impales himself on his sword. 
In the last scene of the tragedy, Brutus realizes that he and his remaining servants will be captured. He eventually convinces Strato to hold his sword for him as he impales himself on his own sword.