What makes Friar Lawrence not explain the whole thing before the things go too far in Act IV of Romeo and Juliet?
Since you are talking about Act IV, I assume that you are just wondering why Friar Lawrence doesn't explain that Romeo and Juliet love each other and are married, rather than going through with the whole charade that he concoct where she'll drink the potion and appear dead.
I would suggest the following:
- He doesn't understand how likely the plan is to fail. He thinks Romeo will go to Mantua and stay there and so little immediate harm can come of the deception.
- He knows that both the Montagues and Capulets would be outraged by the marriage and his complicity in it. The two families are influential and it may be that he would fear the consequences of having them both angry at him.
That's my opinion. Anyone else?
I agree with pohnpei397 on the second point. Churches and religious officials of the time were supported almost entirely by the "charity" and donations of the upper class. To have the Montagues and/or the Capulets angry with the Friar would seriously diminish his income.
I would like to add another opinion/possible answer to the question. I do believe that he was a true friend to Romeo and did honestly want to see Romeo happy. He would have chosen, therefore, to keep quiet about the marriage because he did not want to jeopardize Romeo and Juliet's future.