It's hard not to feel like we're sitting across from Dorothy Parker listening to her talk when we read "One Perfect Rose." Phrases like "all tenderly," "do you suppose," and "Ah no, it's always just my luck" are phrases we use more when talking casually than when writing poetry.
So, conversational tone? Check. But this poem doesn't sound like any old conversation. Nope. It's a conversation that is also a little song, complete with rhymes, refrains ("One perfect rose… One perfect rose… One perfect rose"), and alliteration ("Long love" in line 7)—things we associate with poetry.
It's as if the poem is using traditional poetry sounds, but also trying to break through and reach the reader on a more informal level at the same time. Hmm! That sounds a lot like our speaker, doesn't it? She's tired of the old ways (rose = love: yawn), and looking for a more meaningful connection. In this sense, then, the poem's use of sound really reflects what this poem is all about.