Romeo and Juliet are young... she's almost 14, his age has been assumed to be anywhere from 17 to 26. When teenagers experience what they think is love, it is usually infatuation, right? I think just as the other editors believe that the Friar is encouraging these guys to slow down a little bit. There is so much more that goes into a relationship than the physical experience. The friar is warning them to be aware that there are more aspects to a love relationship worth working through. For example, companionship, entertainment, working together, parenting children, and building each other up are aspects of a relationship both parties grow from. These are the characteristics that create a lifelong relationship. This is ironic because their relationship ends up being so short even after this carefully worded admonishment.
In this scene, Friar Laurence is about to marry Romeo and Juliet. He and Romeo are talking when he says the lines you cite. He is telling Romeo that it is not a very good idea to be rash and passionate.
Your lines,in modern English, mean something like "be mellow (moderate) in your love for each other and you'll be able to love each other for a long time (long love doth so).
Before this, the Friar has been telling Romeo that too much of a good thing (too much honey, for example) tends to end up being bad. So too with love -- he wants Romeo to be more mellow about it so that the love will last a long time.
I hope that helps...
In Act Two, scene six of William Shakespeare's Romeo and Juliet, Romeo is impatiently waiting the arrival of his bride-to-be. The scene takes place in Friar Laurence's cell. Romeo tells Friar Laurence that he does not care what sort of misfortune might stem from his marriage to Juliet, the daughter of his family's sworn enemy. It can only pale in comparison to his love for Juliet and their happiness together. This is when Friar Laurence interjects with the particular quote you are asking about. What he is telling Romeo is that he should not love Juliet so intensely, for intense love dies quickly. If he loves her moderately and with less passion, the love they share for one another will last longer. Juliet enters thereafter.