Provide evidence for why Friar Laurence did what he did.

Friar Laurence is a pivotal character in Romeo and Juliet, and without him, much of the plot of the play could not move forward. In addition to marrying the star-crossed lovers, the friar is also the procurer of the sleeping potion that leads, through a series of unexpected events, to Romeo and Juliet's deaths. Friar Laurence means the lovers no ill will, but his attempts to help them, shrouded in lies and secrecy as they are, enable tragedy to occur.
One might not expect a religious man like the friar to trade in subterfuge, but he seems to have a moral outlook that accounts for sometimes doing seemingly suspect things for good reasons. In agreeing to marry Romeo and Juliet in act 2, Friar Laurence hopes to see the feuding Capulets and Montagues make peace as a result of their union. When the two are wed, their problems do not end, however. By act 3, Romeo has been exiled from Verona for killing Tybalt, and Juliet is set to marry Paris against her will. Friar Laurence steps in once again to aid the lovers.
As pressure to marry Paris mounts in act 4, Juliet confesses to Friar Laurence that she will kill herself if her parents force her to go through with it; at seeing her draw a dagger, the friar quickly devises a plan:

Juliet:
Give me some present counsel, or, behold,'Twixt my extremes and me this bloody knifeShall play the umpire, arbitrating thatWhich the commission of thy years and artCould to no issue of true honor bring.Be not so long to speak. I long to dieIf what thou speak’st speak not of remedy.
Friar Laurence:
Hold, daughter. I do spy a kind of hope,Which craves as desperate an executionAs that is desperate which we would prevent.If, rather than to marry County Paris,Thou hast the strength of will to slay thyself,Then is it likely thou wilt undertakeA thing like death to chide away this shame,That copest with death himself to ’scape from it.An if thou darest, I’ll give thee remedy. (4.1.63–78)

Once again, the friar acts out of a worthy impulse—to save the life of Juliet and prevent her from the grievous act of suicide—but his solution is fraught with danger. Romeo and Juliet are impulsive in their behavior throughout the play, and they often act without foresight. Unfortunately, Friar Laurence is not so different. He devises a plan far too quickly, and his secretive maneuvers to aid the two lovers lead to a tangle of lies and confusion.
The friar concocts a sleeping potion to make Juliet appear as though she has died, thus preventing her from marrying Paris and allowing Romeo the opportunity to spirit her away. The message that Friar Laurence sends to inform Romeo of the plan does not reach him, however; the bearer of the missive, a fellow friar, is delayed. Romeo receives news only of the deception—that Juliet has died—and is not aware of the sleeping potion and his intended part in the plan. As a result, Romeo returns to Verona, mistakes Juliet for dead, and poisons himself. Upon waking, Juliet finds her lover dead and kills herself in turn.
Friar Laurence does not account for the potential for confusion and chance in his schemes, and whether it be carelessness or pride that prevents him from seeing these critical flaws, his plans do more harm than good. While the families do make amends at the play's end, as the friar had hoped, there is a tragic cost of life that Friar Laurence has played an unintentional but undeniable part in. He sought to save the life of a despondent Juliet by reuniting her with Romeo, but his interference ultimately could only unite them both in death.

After Romeo instantly falls in love with Juliet after seeing her at the Capulet ball, he expresses his emotions to her in the famous balcony scene. In act 2, scene 3, Romeo visits Friar Lawrence's cell, and the friar can immediately tell that Romeo has been up all night. Friar Lawrence initially thinks that Romeo has been with Rosaline all night but is astonished to learn that Romeo no longer has feelings for her and is instead attracted to his family's enemy. After Friar Lawrence criticizes Romeo for his capricious nature and fleeting emotions, he agrees to secretly marry Romeo and Juliet. Friar Lawrence justifies his decision to marry Romeo and Juliet by saying,


Thy love did read by rote, that could not spell. But come, young waverer, come, go with me, In one respect I’ll thy assistant be, For this alliance may so happy prove To turn your households' rancor to pure love (Shakespeare, 2.3.88-92).


Essentially, Friar Lawrence agrees to marry Romeo and Juliet because he thinks their marriage can possibly bring an end to their families' longstanding feud.