He states that he believes in moderation, and that extremes are dangerous. In this scene, he tries to convince Romeo and Juliet not to get married, because he believes that it would be too extreme. He also tries to convince them to wait, because he believes that it would be better for them to marry later, when they are older and more mature.
However, when Romeo and Juliet insist on getting married, Friar Lawrence agrees to help them. This shows that he is willing to make an exception to his philosophy in order to help the people he cares about.
Friar Lawrence's actions in this scene also show that he is a compassionate and understanding person. He knows that Romeo and Juliet are in love, and he wants to help them be happy. He is also willing to risk his own reputation in order to help them, which shows that he is a truly good person.