He's a coward. He heard the watch coming and didn't want to be caught in there.
The friar leaves Juliet alone in the tomb because he hears someone approaching and fears being caught. He hopes that Juliet will wake up soon, but he ultimately leaves her to avoid being implicated in the tragedy.
Juliet refused to go.
because he thinks he hears the capulets coming from outside and wants to escape before they find him
Friar Laurence finds Romeo, who believes Juliet is dead, in the Capulet tomb. Friar Laurence arrives to find Romeo devastated over Juliet's apparent death and helps Romeo take action to be with her.
Romeo tells Balthasar that he is going into the tomb to retrieve a ring he left on Juliet's hand, so that Balthasar will leave him alone at the tomb.
Because he heared the noises from outside of the tomb.
Love.
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.
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.
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.
Friar Laurence devises a plan for Juliet to fake her own death by taking a potion that will make her appear lifeless. Juliet is initially hesitant, fearing it may not work, or that she will wake up alone in the tomb and go mad. She is also concerned about the consequences if the plan fails, such as being buried alive or waking up to find herself in the tomb with no way to communicate with Romeo.