Throughout the play, characters make mistakes because they rush into things. Friar Lawrence is reminding us that if you act too hastily, disaster can result.
Romeo and Juliet fall in love way too fast.
As Friar Lawrence points out, Romeo loved one girl one day and another girl the next. Romeo and Juliet met at the party and that was it, they were suddenly in love.
Holy Saint Francis! What a change is here!
Is Rosaline, that thou didst love so dear,
So soon forsaken? (Act 2, Scene 3, enotes etext p. 46)
Friar Lawrence is surprised that Romeo can love so passionately, so quickly. This surely spells disaster.
Romeo interrupts Mercutio’s fight, then fights Tybalt.
If Romeo had stopped to think about it, he would not have been anywhere near that fight. He got in the way and Mercutio was hurt.
Why the devil came you between
us? I was hurt under your arm. (Act 3, Scene 1, p. 63)
If Romeo had acted more throughtgully, Mercutio might not have been killed. Romeo also acted too hastily in fighting Tybalt, killing him and getting banished for it. This contributes to the chaos and tragedy.
Juliet’s parents should not have been in such a hurry to get her married.
Juliet is young. If her parents had not been in such a hurry to marry her off, she would not have had to take the sleeping potion to fake her death, and Romeo would not have thought her dead and killed himself.
I wonder at this haste, that I must wed
Ere he that should be husband comes to woo. (Act 3, Scene 5, p. 82)
Juliet’s parents’ haste caused not only her death, but Romeo’s as well.
In each of these cases, things could have been solved much more easily if people had waited and stopped to think before acting too hastily.