Friar Lawrence's monologue that begins in Act II, Scene 3, just before Romeo comes to see him, shows how Shakespearean characters can represent enduring truths about humans and society. During his monologue, the friar is collecting plants that are medicinal as well as plants that are poisonous, and he describes how each flower that seems evil has some good properties or applications, while a flower that seems good could also be put to a negative use.
We can read the friar's discussion of plants as symbolic of humans and human actions and emotions; just like plants, they all possess great power to do harm or good. He says, "Virtue itself turns vice, being misapplied, / And vice sometime by action dignified" (Act II, Scene 3, lines 21-22). In other words, good can turn bad when it is poorly managed, and what seems bad can, when applied, actually be put to good use. We see the way the love between Romeo and Juliet is mismanaged, giving it the power to damage and wound, which ultimately hurts both families. We typically think of love as a good thing, but it turns bad in this play.