What is Friar's advice to Romeo and Juliet? How does this foreshadow the end of the play?

The Friar's advice.....

Therefore love moderately; long love doth so;
Too swift arrives as tardy as too slow.

This foreshadows the deaths of the two lovers..... senseless deaths that could have been avoided had they loved moderately and thought before acting..... rather than rushing head long into tragedy because they loved too fast and acted too quickly.