Shakespeare's play Romeo and Juliet is known as being the story of star-crossed lovers who meet a tragic end. Anyone familiar with the events surrounding the young lovers knows that a great deal of the disaster that befalls them is based on timing and a long-standing feud between the Capulets and the Montagues, though the original cause of that feud has grown obscure over time.
At the beginning, Romeo spends his time bewailing his unrequited love for Rosaline. Instead of returning his love, she has promised herself to the Church. Rather swiftly (too swiftly some believe), Romeo meets Juliet and they are instantly drawn to each other. After only a short while, they speak of love and marriage even with the certainty that their families will never agree.
The Friar tells Romeo:
Young men's love then lies
Not truly in their hearts, but in their eyes. (II.iii.68-69)
The Friar's words echo those found Shakespeare's A Midsummer Night's Dream:
Love looks not with the eyes, but with the mind;
And therefore is wing'd Cupid painted blind. (I.i.239-240)
These quotes generally put forth that love for young men is something that is not born of the heart but of what he sees!
One can comprehend the Friar's concern in that Romeo seems not to know his own heart or does not truly know what love is—or both. The Friar cannot understand how Romeo fell out of love with Rosaline (someone that the Friar warned him about—not in her person but in his infatuation) only to immediately fall in love with another—and a Capulet on top of everything! The change in Romeo is too mercurial to lend itself to the priest's peace of mind and so it fair that the Friar might not trust Romeo's decision. Friar Laurence continues to declare that women will not be faithful if their men are not strong.
Pronounce this sentence then:
Women may fall when there's no strength in men. (80-81)
In making this statement, the Friar seems to be throwing up his mental hands in frustration, stating that the world has gone mad: for if young men cannot be strong, then the "frailer" woman does not have a chance. Because Romeo appears so impetuous, one can only imagine that he does not know that he is doing. Arguably, if he leaves Rosaline to so quickly fall in love with Juliet, could he not do the same again with another woman that he finds beautiful?
It would seem that Friar Laurence's apprehension is well-founded. The only thing, however, that does not bear out his concerns is Romeo's undying devotion to Juliet. Perhaps he was simply infatuated with Rosaline, but his love for Juliet appears to be genuine in that he is prepared to leave all he knows and loves to spend the rest of his life with young Juliet. In the end, living without her is something he cannot face. To Juliet's [sleeping form], Romeo promises:
I still will stay with thee
And never from this palace of dim night
Depart again. (V.iii.106-107)
While at the start Friar Laurence appeared to have good reason to be concerned about Romeo's love for Juliet, it seems that his trepidation was groundless, as Romeo promises never to leave Juliet's side.