It's act 3, scene 3 of Romeo and Juliet, and Romeo is in a despondent mood. He's sad because Prince Escalus has just banished him from Verona for killing Tybalt in a street brawl.
On the whole, one might think that Romeo would be mightily relieved for having avoided the death penalty. The young man is incredibly lucky he won't have to pay for his life for killing Tybalt. But Romeo doesn't see it that way; banishment from Verona means being separated from his beloved Juliet, the love of his life.
The very thought of being away from Juliet is so unbearable that he tells Friar Laurence that he wants to kill himself. The Friar responds by telling Romeo that in killing himself, he will also be killing Juliet, the woman who shares his life:
Wilt thou slay thyself,
And slay thy lady that in thy life lives
By doing damned hate upon thyself?
Suicide was regarded as a serious sin in those days, and the friar, as a man of God, naturally wants to remind Romeo of this.
In his lengthy monologue, he also tries to talk Romeo out of all thoughts of suicide by arguing that if Romeo does kill himself, then the love he claimed to have for Juliet will have been shown to be nothing but a lie. For good measure, he will be killing the very love he vowed to cherish:
Thy dear love sworn but hollow perjury
Killing that love which though hast vowed to