In Act 2.6 of "Romeo and Juliet", what weakness in Romeo does Friar Lawrence point out before marrying him to Juliet?
These violent delights have violent ends
And in their triumph die, like fire and powder,
Which, as they kiss, consume. The sweetest honey
Is loathsome in his own deliciousness
And in the taste confounds the appetite.
Therefore love moderately: long love doth so;
Too swift arrives as tardy as too slow.
There's Friar Laurence's advice, and it might be used as a sort of "tagline" for the play itself. Friar Laurence warns Romeo that if you rush into things in such a violently passionate way ("violent delights") and immerse yourself in the pleasure of them, then it will end violently.
It's like, he says, honey that is too sweet, and so is too nice tasting and makes you not want to eat it. It's like fire and powder, which work so well together that they explode and destroy both of the original ingredients.
Therefore, Romeo should love moderately, love in moderation. Long love, long-lasting love, does that.
Does Romeo take the advice? No. And what happens? Exactly what Friar Laurence suggests. The two lovers race in, and like fire and gunpowder, destroy themselves.
Friar Lawrence basically tells Romeo to cool his jets, a little. He reminds Romeo that he should love moderately, and love for longevity. He remembers how easily Romeo has fallen in and out of love in the past, and warns Romeo that to proceed without caution is to face doom. After all, to rush into a marriage with the daughter of his enemy would be dangerous if it weren't backed up with strong, true feelings of love. Friar Lawrence knows that Romeo falls hard and fast, but often falls just as easily out of love again. He doesn't want that to happen with Juliet, because he is counting on their marriage to unite the two families.