In the play Julius Caesar by William Shakespeare, why does Brutus think he must commit suicide?Julius Caesar by William Shakespeare

In Act V, Brutus and Cassius quarrel; Cassius thinks that
their troops should rest and wait for their enemy to march upon them while Brutus
insists that they engage the enemy at Philippi.  However, this taking  battle early with
the troops of Octavius, Lepidus, and Marc Antony, the triumvirate, proves to be a
tactical error, and the troops of Brutus and Cassius are defeated.  Brutus calls
together the few survivors, and tells them to rest; he then asks Clitus to kill him, but
his servant replies, "Iill rather kill myself."  As Brutus speaks to himself, Dardanius
and Clitus overhear him say,

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 Why, this, Volumnius:
The ghost of
Caesar hath appear'd to me(20)
Two several times by night; at Sardis once,

And this last night here in Philippi fields.
I know my hour is
come.

Brutus is defeated and
bids farewell to the others and expresses his great regret that his final hour is so.
Finally, Brutus retains Strato to assist him in his suicide, for he would rather die
than be put in bondage or surrender.

When Brutus is
defeated and dies, Antony and Octavius arrive to eulogize him.  Marc Antony calls him
"the noblest Roman of them all while Octavius Caesar issues the order to give Caesar the
"respect and rites of burial."