When Romeo goes to see Friar Lawrence early in the morning after meeting Juliet at the Capulet party, it is with the plan to ask Friar Lawrence to marry him and Juliet. When he mentions this to the Friar, the Friar is rightfully shocked - he points out that he can still see the stain of an old tear Romeo shed for Rosaline on his cheek and can still hear his lovesick groans in the air. Just yesterday, he was in love with Rosaline. And yet, now he wants to marry Juliet?
Initially, it seems Friar Lawrence will not agree to the plan - but then he thinks about the family feud and recognizes that this marriage might be just what is needed to put an end to the families fighting. He agrees to perform the marriage - but he also advises Romeo to proceed cautiously, saying, "Wisely and slow; they stumble who run fast." He is concerned that Romeo and Juliet barely know each other and have not had time to let their love grow. They should slow down and be sure about their feelings and decisions so they don't take a wrong step.
Friar Lawrence later, at the wedding, advises both of them: "Love moderately; Long love doth so." He also warns, "These violent delights have violent ends." Both of these statements point out that extremes of emotion do not serve for the most stable relationship or future - for their love to last a long time, Romeo and Juliet need to find a way to contain these extremes or they will be consumed.
Ironically, while he advises them to go slow, he agrees to marry them later that day. Romeo and Juliet, caught up in their passions, cannot be expected to think completely rationally, but he should be. Because he does not - and instead helps them to move too fast - they "stumble" and have "violent ends." For this reason, as well as for other actions he takes later in the play, he deserves much of the blame for Romeo and Juliet's tragic ends.