Symbolic analysis of the asphodel and wormwood line
"I actually considered this scene [the first Potions lesson] the first clue in the series that Snape had loved Lily Evans, and my personal favorite. The quote in my signature is a one-sentence statement of Snape's life story in symbolic terms. Asphodel, a Lily Family plant, is associated in mythology with the dead. Wormwood is a plant known for its bitterness. So as I see it, Rowling is telling us here that Snape loved and lost Lily (asphodel), and the bitterness of his knowledge that he bears responsibility for that (wormwood) has made his life like a living death."