What happens
At dawn Friar Lawrence gathers herbs and muses that every plant holds both medicine and poison, depending on how it is used. Romeo arrives, plainly not having slept. The Friar assumes he has been with Rosaline and is startled to hear that Rosaline is forgotten — Romeo now loves Juliet and wants to marry her that very day. The Friar scolds him for how quickly young men's affections move, reminding him how recently he wept for someone else. Still, he agrees to perform the marriage, hoping the union of a Montague and a Capulet might turn their families' hatred into love.
Why it matters
The Friar's opening speech about herbs — "Within the infant rind of this small flower / Poison hath residence, and medicine power" — is the play's thesis spoken in a garden. The same things heal and kill: love, haste, the very potion he will later mix. Keep this speech in mind, because the play keeps proving it.
He agrees for a good reason and a bad one. The good: he genuinely hopes to end a deadly feud. The bad: he is persuaded by a teenager who fell in love yesterday. The Friar is the adult who could slow all this down, and instead he speeds it up. His own warning — "they stumble that run fast" — is advice he then ignores.
Romeo's instant switch from Rosaline to Juliet is meant to unsettle us. Is this the real thing, or the same infatuation wearing a new name? The Friar asks exactly that question. The play's answer is complicated: the feeling is genuine, and it is also dangerously quick.