The repeated "fl" sounds are soft/fun and therefore evoke a lighthearted feeling.
Explanation:
This is an example of assonance, by technicality, but can also be classified as alliteration -- the repetition of the same first letter(s) in a series of words. The repeating sound provides not only a bit of rhyme but rhythm as well. If Poe wished to do by contrast, there would be a bit of irregular rhythm which in turn personifies someone's mind "stumbling", figuratively "falling down stairs". But since this is not the case, the opposite is true. It makes readers feel cheery because it, musically speaking creates a <u>fl</u>uffy and upbeat rhythm. (yes, I purposely used that word choice)
