“Young men’s love then lies/Not truly in the hearts, but in their eyes”
Then young men love with their eyes, not with their hearts. Friar Lawrence is saying that Romeo is fickle (easily changed) and only “loves” Juliet for her appearance since he has so quickly forgotten about his “love” Rosaline.
“Wisely and slow. They stumble that run fast.”
Go wisely and slowly. Those who rush stumble and fall. Friar Lawrence is warning against impulsivity/haste in falling in love and getting married.
“So smile the heavens upon this holy act That after-hours with sorrow chide us not.”
May the heavens be happy with this holy act of marriage, so nothing unfortunate happens later to make us regret it. Friar Lawrence hopes that the heavens will be happy with this decision to marry Romeo and Juliet and that nothing will happen in the future to cause him to regret marrying them.
“These violent delights have violent ends”
These sudden joys have sudden endings. Friar Lawrence is concerned that the impulsivity and suddenness of the love/wedding will have a sudden (and violent) ending.
“For this alliance may so happy prove To turn your households’ rancor to pure love.”
This marriage may be lucky enough to turn the hatred between your families into pure love. Friar Lawrence is willing to perform the wedding in the hopes that the hatred between the two families will end.