The friar seems to have some doubts about the wisdom of what he is about to do. romeo responds that death can do what it dares on him after he is married to juliet. what reservations does the friar express?

The friar is trying to say that a love that begins quickly will end quickly.
He believes that Romeo and Juliet are rushing things by wanting to get married at such a young age, especially after knowing each other for such a short period of time. He is afraid that something bad is going to happen if they rush things, and it did - they both ended up dead.
This is what he says:
"These violent delights have violent ends."