Friar Laurence is the voice of reason. So far, we've seen the Capulet and Montague men fight in the streets because of an old feud; we've listened to the mindless and annoying banter of Juliet's nurse; we've watched Romeo fall in love with Juliet from across the room, then woo her on her balcony and decide before the night is through that he is madly in love with her and will marry her. Almost all discourse up to this point has been driven more by emotion and empty-headedness than by reason, and we—the audience—need reason. Our patience, by this point, has been strained enough.
When we first see the friar, he is musing to himself that all good contains the possibility of bad, and all bad contains the possibility of good. He sees the world in balance. He also points out that Romeo is crazy to be "in love" with Juliet not a day after he was pining for Rosaline.
Friar Laurence's reason leads him to believe that the cure for what ails the Capulets and Montagues is the marriage of their children, which advances the plot. This proves to be too simplistic a cure, as has already been noted below.