Friar Lawrence's remarks about flowers and herbs underscore the moderation vs. excess theme of "Romeo and Juliet." As he gathers these flowers and herbs in his soliloquy, he reflects,
For naught so vile that on the earth doth live,/But to the earth some special good doth give/Nor aught so good but, strained [turned aside] from that fair use,/Revolts from true birth [its true function], stumbling on abuse [misuse].Virtue itself turns vice, being misapplied,/And vice sometime's by action dignified.
Friar Lawrence notices that a poisonous plant can produce a cure if only a small amount of its poison is used. Likewise, if something good is overused, its effects are mitigated. In his reflections, he considers that man, too, possesses both good and evil; if he allows his evil side to be stronger, then the beauty of his soul will be destroyed: "Full soon the canker death eats up that plant." Moderation is the key to controlling evil from taking hold.
Later, when Romeo rushes in and tells the friar that he is in love with Juliet and wants to marry her, the priest cautions Romeo with the same idea of moderation in mind: "Wisely and slow. They stumble that run fast."
Ironically, however, Friar Lawrence does not always follow his own advice. In Act Four when Juliet rushes to Friar Lawrence in her despair, he devises a "plan" for her to take a drug, appear dead but later wake to the joy and forgiveness of her parents. However, because he enters hastily into this plan without considering extenuating factors involving the always impetuous Romeo, fate is allowed to work and "the worser is predominant" and "death eats up that plant"--death takes both lovers.