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.