Friar Lawrence gives Juliet the potion as part of his plan for Juliet and Romeo to reunite and as a way for Juliet to avoid going through with her wedding to Paris, according to About.com's Shakespeare section. Friar Lawrence wants to bring an end to the feud between the Capulets and Montagues and hopes that a union between Romeo and Juliet accomplishes this.
Romeo and Juliet each consult Friar Lawrence at different times during the play, making him a confidante of them both, and he performs their secret marriage. When Romeo is exiled and Juliet is betrothed to Paris, she panics and seeks Friar Lawrence's advice. He designs a plan that involves using a mixture of herbs designed to make Juliet appear dead.
His plan is to notify Romeo of what is happening and to tell him to be at the tomb when Juliet wakes from her sleep. Romeo, however, does not get the Friar's letter. He goes to the tomb, where takes his own life because he believes Juliet to be dead, and he dies just before Juliet awakens.
In the meantime, Friar Lawrence feels that something is wrong and goes to the tomb to find everything gone awry. Juliet wakes up, and the Friar tries to convince her to come with him because the authorities are coming. She refuses, and he, unwilling to face the consequences of his involvement, runs away. Juliet kills herself. The Friar is apprehended and confesses his part in what has happened. He takes responsibility by saying, "if aught in this/ Miscarried by my fault, let my old life/ Be sacrificed, some hour before his time,/ Unto the rigour of severest law."