Because he knows that Juliet is married to Romeo. He was the one who married them.
Romeo wants the Friar to marry him to Juliet.
Romeo proposes to Friar Laurence that he and Juliet be married in secret, without the knowledge of their feuding families. Romeo seeks the Friar's help in marrying them to bring peace between the Capulets and Montagues.
Romeo goes to friar Lawrence about marrying Juliet
Friar Laurence hopes that his plan for Juliet to fake her own death will allow her to avoid marrying Paris and ultimately reunite with Romeo. He believes this will bring peace and happiness to both Juliet and Romeo, ending the feud between their families.
Friar Laurence married off Romeo and Juliet.
Friar Laurence is the friar who marries Romeo and Juliet.
Romeo asks friar Laurence to marry him and Juliet in secret
Juliet asks the Nurse to listen to her confession instead of Friar Laurence.
Friar Lawrence married Romeo and Juliet in his cell.
friar Laurence
The Nurse and Friar Lawrence knew about it before anyone else because the Friar performed the marriage and the Nurse was Juliet's close confidant so she told her everything.
Friar Laurence confesses to Friar John at the end of Romeo and Juliet. He reveals the truth about his role in the tragic events that led to the deaths of Romeo and Juliet.