At the end of Act II, scene iii, just after he has agreed to marry Romeo and Juliet, Friar Lawrence offers Romeo the following advice:
This line foreshadows most of the rest of the plot of the play concerning Romeo and Juliet as they each violate this advice time and time again. At almost every turn both Romeo and Juliet act impulsively without thought and risk their own lives and likely the lives of others. At the play's end, their emotional impulsivity gets the better of them when they each take their own life rather than thinking through the facts and evidence that reside in front of them. These are the tragic flaws of both Romeo and Juliet and Friar Lawrence comments on them very early in the play, foreshadowing their demise.
As Romeo and Friar Laurence wait for Juliet to arrive so the young lovers can be married, the Friar utters a blessing on the marriage, saying, "So smile the heavens upon this holy act/ That after-hours with sorrow chide us not (2.6.1-2)!" Romeo has an arrogant response to this blessing, taunting sorrow and saying he can face anything as long as he has Juliet. Once he marries Juliet, he believes everything will be all right. When he scolds Romeo for this foolishness, the Friar says, "These violent delights have violent ends (2.6.9)."
With this line, and the small speech that follows it, the Friar is warning Romeo that much damage can come from someone being too eager and too wrapped up in a good thing. He cautions Romeo to "love moderately (2.6.14)" in order to avoid any trouble. He has no idea, of course, just how important this warning is, or how completely Romeo will ignore the advice. The tragic events to follow happen as a result of the rashness and all-consuming love of both Romeo and Juliet, eventually leading to their own "violent ends". If Romeo had heeded the Friar's words and slowed down a bit, both Romeo and Juliet might have thought more clearly about their actions and found a better way to proceed.
At critical points in the play, Friar Laurence advises restraint and cool consideration, though neither Romeo nor the other characters in the play are terribly good at either. “Wisely and slow, they stumble that run fast,” he tells Romeo after seeing Romeo practically bounce with joy at his agreement to marry Romeo and Juliet. Then three scenes later he cautions, “These violent delights have violent ends.” Romeo in particular proves the friar right.
Later in the play Romeo jumps into the duel between Mercutio and Tybalt, and inadvertently causes Mercutio’s death: “Why the devil came you between us?” scolds Mercutio, “I was hurt under your arm!” Romeo’s headstrong behavior, though well intended, causes a fatality. Later on, in Juliet’s tomb, distraught at seeing what he has every reason to believe is her dead body, he poisons himself, just moments too soon – Juliet wakes up almost as soon as he dies. Tragically, in both cases, he lets his storm of emotions “run fast”, and both times they lead to “violent ends”.
Friar Lawrence warns Romeo, "Wisely and slow, they that run fast stumble." He gives this warning after agreeing to marry Romeo and Juliet. He is essentially warning against the hastiness of their union. They have only just met, and he is cautioning them against moving too quickly.
Ultimately, his warning foreshadows their doom. Romeo's tendency to act without thinking first directly leads to the death of both characters. The friar states:
"These violent delights have violent ends. Which in their triumph die, like fire and gunpowder, which as they kiss, consume." (2.6.9-11).
The violent delights, or the love, of Romeo and Juliet, often have violent ends, in this case death. While their love may be beautiful and powerful, it is also brief and destructive. Thus, with his words, the Friar unknowingly foreshadows the passionate but brief love of Romeo and Juliet, which ends in death.
Friar Lawrence chides Romeo for forgetting Rosalin so easily and falling for Juliet so quickly. In regards to Romeo and Juliet's marriage, Friar Lawrence says, "These violent delights have violent ends." (2.6.9). This warning from the Friar foreshadows that Romeo and Juliet's marriage will end as quickly and as "violently" as it began during the party in Act I.