Why's 'Cellar door' the most beautiful English word/ phrase?

CLARIFICATION: Tolkien is considered by many (including me) the author of this observation.

There are definitely some comments above, and it's only my OCD need to not have unanswered questions on this board that compels me to answer, but here goes:

I think it is the combination of two factors: a smooth ellision of vowel sounds from one to another, and that that ellision is downward.

First of all, consider that Tolkien is (most likely) referring to a posh British vowel pronunciation. A nasal, Midwestern American pronunciation of "cellar door" is positively grating. So here's what we're looking at:

SEH - LAH - DOH

If you look at yourself in the mirror pronouncing this, your jaw moves smoothly downward as the phrase progresses. If you reverse the order ("Duracell" is a fair approximation), you have the same progression, but in the opposite direction, and it's not as pleasing...I can't say precisely why.

Ultimately, I think we're stepping into the murky waters of evolutionary psychology, but I think that at least part of the appeal of "cellar door" is the smooth transition of vowel sounds, and the fact that the consonants don't get much in the way.

'Cellar door' is considered the most beautiful word because of phonaesthetics. It's the study of beauty associated with speech sounds.

Words perceived as euphonious/pleasant tend to have a majority of a wide array of criteria; here are some major ones:

• Contains two or more syllables (e.g. Cellar door -> SEH-LAH-DOH)
• Stress on the first syllable (Cellar door -> SEH-lah-doh)
• Have /l/ sound usually followed by /m, s, n, k, t, d/. (Example: luminous contains the first four i.e. /l/, /m/, /n/, /s/, cellar door has three i.e. /s/, /l/, /d/)
• Short vowels (e.g., the schwa, followed in order by the vowels in bid, bed, and bad) are favored over long vowels and diphthongs (e.g., as in bide, bode, bowed).
• Three or more manners of articulation (with approximant consonants the most common, followed by stop consonants, and so on).
• The consonants and vowels vary from syllable to syllable. (Examples: Cellar door -> /'sɛlədɔ:/, melody -> /'mɛlədi/, luminous -> /ˈluːmɪnəs/).

_{[Adapted from Wikipedia and Saving our Prepositions: A Guide for the Perplexed - Google Books]}

Example from "The Ugliest Words" by David Crystal:

"Here's an experiment. You're in a spaceship approaching a planet. You've been told there are two races on it, one beautiful and friendly to humans, the other unfriendly, ugly and mean-spirited. You also know that one of these groups is called the Lamonians; the other is called the Grataks. Which is which?

"Most people assume that the Lamonians are the nice guys. It's all a matter of sound symbolism. Words with soft sounds such as /l/, /m/, /n/ and long vowels or diphthongs, reinforced by a gentle polysyllabic rhythm, are interpreted as 'nicer' than words with hard sounds such as 'g' and 'k,' short vowels and an abrupt rhythm." (David Crystal, "The Ugliest Words." The Guardian, July 18, 2009)

Origin of 'cellar door':

The origin of cellar door being considered as an inherently beautiful or musical word is mysterious. However, in 2014, Nunberg speculated that the phenomenon might have arisen from Philip Wingate and Henry W. Petrie's 1894 hit song "I Don't Want to Play in Your Yard", which contains the lyric "You'll be sorry when you see me sliding down our cellar door". Following the song's success, "slide down my cellar door" became a popular catchphrase up until the 1930s or 1940s to mean engaging in a type of friendship or camaraderie reminiscent of childhood innocence. A 1914 essay about Edgar Allan Poe's choice of the word "Nevermore" in his 1845 poem "The Raven" as being based on euphony may have spawned an unverified legend, propagated by syndicated columnists like Frank Colby in 1949 and L. M. Boyd in 1979, that cellar door was Poe's favorite phrase.

Tolkien, Lewis, and others have suggested that cellar door's auditory beauty becomes more apparent the more the word is dissociated from its literal meaning, for example, by using alternative spellings such as Selador or Selladore, which take on the quality of an enchanting name (and both of which suggest a specifically British pronunciation [non-rhotic accents] of the word: /sɛlədɔː/).

[Phonaesthetics - Wikipedia]