Friar Laurence wants peace between the Montagues and Capulets, and this guides his decisions. He also tries to protect Romeo from making an unrecoverable mistake in his young love, hormone-driven as it is. In Act II Scene 3, Romeo goes to see Friar Laurence to ask him to wed himself and Juliet. At first, Laurence thinks he wants help with Rosaline, the woman who had spurned his love before he met Juliet. Friar Laurence had warned Romeo against Rosaline. Romeo says, "Thou chid'st me oft for loving Rosaline," to which the friar replies: "For doting, not for loving, pupil mine." Laurence is therefore guiding Romeo to make wise decisions, to think despite what his heart tells him.
He also wants peace between the feuding clans. He says, "In one respect I'll thy assistant be; / For this alliance may so happy prove, / To turn your households' rancour to pure love." He believes that the marriage of Romeo and Juliet will end the senseless feud and bloodshed, and thus is happy to marry them.
Laurence takes care of Romeo like a son. When Romeo kills Tybalt (violating the Prince's direct order), Laurence hides Romeo in his cell. When Romeo learns that Juliet mourns the loss of both Tybalt and Romeo, Laurence convinces him to "man up" and appreciate that he is alive and only banished; Romeo is to go live in Mantua and wait until the friar can beg pardon of the prince, reconcile the families, and bring Romeo back. Laurence is, in all things, working behind the scenes to make peace and save lives.
When Juliet's father arranges a hasty marriage between her and Paris, Laurence hatches the plan to have her drink a potion that will make her appear dead, so she'll be "buried" in the family tomb and await Romeo--and also to save her life, since she has threatened to kill herself if she has to marry Paris. Laurence promises to send a letter to Romeo in Mantua with the plan.
He cannot foresee that Friar John will not deliver the letter in time because of a quarantine, and that the news would reach Romeo by other means first, of course.
His motivations to help everyone complicates not just his character but the plot. He tries to be everything to everyone, always doing what he feels is best to correct the wrongs and protect the innocent, but unable to do so in the end. He blames the final outcome on heaven's will.