He's a coward. He heard the watch coming and didn't want to be caught in there.
Juliet refused to go.
because he thinks he hears the capulets coming from outside and wants to escape before they find him
He had to leave, for the "police" was on their way. He had no choice but to go...
that he is going to kill him if he doesn't leave
Because he heared the noises from outside of the tomb.
Love.
Because he knows Juliet is about to wake up and Romeo will not be there, since Friar John was not able to deliver his message.
The Watch. Romeo finds her and thinks she is dead and kills himself. Then Juliet awakes and finds him dead. The Friar returns and tries to convince her to leave the tomb with him but she refuses. The Friar hears the Watch coming and flees the tomb. Juliet stabs herself. The Watch enters the tomb and find Paris, Romeo and Juliet all dead. The Prince, the Capulets, and Lord Montague arrive. Balthasar and the Friar explain what has happened and they all make up.
Juliet was scared of Friar Lawrence's plan because she was afraid it might not work and she would be forced to marry Paris against her will. She also feared the consequences of faking her own death, such as waking up alone in the tomb or being buried alive.
Because the Watchmen are comming.
Friar Lawrence requests a crowbar to open Juliet's tomb and a letter to write to Romeo explaining the situation with Juliet.
The first person to arrive at Juliet's tomb in William Shakespeare's play "Romeo and Juliet" is Friar Laurence, who goes to retrieve Juliet after she has taken a potion to fake her death.