In Romeo and Juliet, who thinks that "young men's love lies not in their hearts,but in their eyes"?

Friar Lawrence says that young men’s love lies in their eyes, not their hearts.

Friar Lawrence is a mentor to Romeo.  He has many poignant things to say to him, because Romeo seems to love very vividly.  He goes from loving Rosaline and pining for her to loving Juliet, almost overnight.  Friar Lawrence believes that it is not true love, but lust. 

Holy Saint Francis, what a change is here!Is Rosaline, whom thou didst...

Friar Lawrence says that young men’s love lies in their eyes, not their hearts.

Friar Lawrence is a mentor to Romeo.  He has many poignant things to say to him, because Romeo seems to love very vividly.  He goes from loving Rosaline and pining for her to loving Juliet, almost overnight.  Friar Lawrence believes that it is not true love, but lust. 

Holy Saint Francis, what a change is here!
Is Rosaline, whom thou didst love so dear,
So soon forsaken? young men's love then lies
Not truly in their hearts, but in their eyes. (Act 2, Scene 3)

Basically, he is saying that if a young man sees a pretty girl, he is in love!  How else could Romeo be sure he loved Rosaline one day, and then sure he loved Juliet the next?  Romeo argues that Rosaline did not love him back, or allow him to love her at least, so it is different with Juliet.  This time, he is really in love! 

ROMEO

I pray thee, chide not; she whom I love now
Doth grace for grace and love for love allow;
The other did not so. (Act 2, Scene 3)

According to Romeo, everything is okay because Juliet shares his affection.  They are in love, and this girl will never break his heart like the last one did!  He is sure of it, even though he has only known her for a few hours.

Friar Lawrence is ready to support this newfound love if it is real. In fact, Romeo and Juliet do get secretly married with his help.  Then he helps Juliet fake her death with a special potion when Romeo is banished for killing Tybalt.  He tries to get a message to Romeo, but the messenger fails, so Romeo does not ever find out that Juliet is not really dead.  Friar Lawrence is knee-deep in the whole thing.  He would have been better off staying out of it.