The Friar is full of wisdom in this scene, which begins with his solilioquy in which he observes that good things, and good actions, may lead to bad ends:
[Nothing is] so good but, strain'd from that fair use,
Revolts from true birth, stumbling on abuse.(20)
Virtue itself turns vice, being misapplied...
Aside from being an important moral lesson, the Friar's comments presage the disastrous results of his own actions, which, while stemming from a genuine desire to help Romeo and Juliet, also, it could be argued, lead to their demise.
The Friar also chides Romeo for moving too quickly from Rosaline, for whom he had only recently professed his undying love, to Juliet. He points to Romeo as an example of how foolish young men can be when they are besotted with a beautiful young woman:
Young men's love then lies
Not truly in their hearts, but in their eyes.
Jesu Maria! What a deal of brine
Hath wash'd thy sallow cheeks for Rosaline!
How much salt water thrown away in waste,
To season love, that of it doth not taste!
And art thou chang'd? Pronounce this sentence then:
Women may fall when there's no strength in men.
Romeo, he is saying, is weak and foolish, and should be strong and judicious when it comes to giving his heart to women. He will continue to counsel Romeo to wait and indeed preaches patience to Romeo throughout the play, (indeed, at the end of the scene he warns Romeo that "they stumble that run fast") though he ultimately indulges the young man by marrying him to Juliet. The Friar's message in this scene, then, is that we should think carefully about what true love is.