he didn't want Juliet to kill herself because she threatened to kill herself if she had to marry Paris. Friar Lawrence also had a realization that both the Montagues and Capulets could finally get along because they've had a long, hateful relationship.
Friar suggests the potion which will put her in a deep sleep, which she can wake up from, rather than killing herself. Juliet needs to appear dead to get out of the wedding with Paris.
