When Friar Lawrence gives his soliloquy, we learn that he knows a lot about plants and nature. Thematically, nothing can be all bad because something good can come from it.
Friar Lawrence seems to be somewhat an expert in plants in their uses, and he is also a philosopher. To him, everything has a purpose.
For naught so vile that on the earth doth live
But to the earth some special good doth give (Act 2, Scene 3)
This is an interesting speech, because Romeo and Juliet’s deaths do lead to something good in that they end the feud between the Montagues and the Capulets. Another important element to the speech is that Friar Lawrence explains that plants have many uses.
Friar Lawrence continues this theme when he basically says that virtue can turn to vice and vice to virtue.
Virtue itself turns vice, being misapplied,
And vice sometime's by action dignified.
This is also significant because he is able to use his skills to make a potion for Juliet so she can fake her death.
Within the infant rind of this small flower
Poison hath residence, and medicine power;
For this, being smelt, with that part cheers each part;(25)
Sometimes things seem to look one way, but turn out to be another. This is a theme throughout the play. The feud between the Montagues and Capulets colors their judgment, as Romeo and Juliet’s passion interferes with theirs. Everything serves a purpose, but some things can serve different ones.