In his own mind, Friar Lawrence is wise to marry Romeo and Juliet because he thinks that the marriage will unite the two families and will bring peace to the town of Verona. In reality it was an unwise and a not very well though out plan to marry the two teenagers. In marrying them, he shared a common trait with both Romeo and Juliet -- he rushed into a decision and did not really think about the repercussions of what was about to be done. Friar Lawrence's decision brought about the deaths of Paris, Romeo, Juliet, Lady Montague, and possibly even the deaths of Tyblat and Mercutio.
In retrospect, it is easy to say that Friar Laurence was unwise to marry Romeo and Juliet considering the unfortunate string of events that follow. At the time, however, the Friar's decision was wise, and would have been a stroke of genius had things worked out as he envisioned. There are three good reasons why it was wise to marry Romeo and Juliet.
First, Romeo convinces Friar Laurence that he is truly in love with Juliet, despite his recent infatuation with Rosaline. Even though he chides Romeo for his quick change of heart, the Friar very much wants to believe that Romeo is telling the truth and that Juliet is the girl for him. Moreover, he must have believed that Juliet felt the same way. Second, because Romeo and Juliet come from two of the most distinguished and wealthy families in Verona, the match makes sense socially. They are both of upper class birth and Romeo seems to be every bit as good a match for Juliet as Count Paris would be. Thirdly, the Friar's primary reason for performing the marriage is to end the bitter feud between the families. He hopes to turn the families' "rancor" to "pure love," and what better way than to marry the two oldest children.
The Friar could have never foretold the miscommunications and pure bad luck which would plague the marriage from the outset. He didn't know that Tybalt was set to challenge Romeo or that Lord Capulet would so rapidly change his mind about Juliet marrying Paris. He was simply attempting to do what he believed was right, not only for Romeo and Juliet, but also for Verona.