Friar Laurence marries Romeo and Juliet because he believes such a marriage would unite the Montague and Capulet families, who had been feuding for many years, perhaps even decades or centuries. He tells Romeo in act 2, scene 4:
For this alliance may so happy prove
To turn your households’ rancor to pure love.
That Friar Laurence keeps the marriage a secret is assumed, even when he is confronted with the debacle following the violence in the streets which claims the lives of Mercutio
and Tybalt and results in the banishment of Romeo. The Friar's solution to Romeo and Juliet
's separation is to keep the marriage confidential until some time has passed and tempers have calmed. He says, in act 3, scene 3:
Go, get thee to thy love, as was decreed.
Ascend her chamber. Hence and comfort her.
But look thou stay not till the watch be set,
For then thou canst not pass to Mantua,
Where thou shalt live till we can find a time
To blaze your marriage, reconcile your friends,
Beg pardon of the Prince
, and call thee...
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