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Exactly, it's how you were taught.

I'm from the "dou" school. It just looks right to me. In fact, my linguistic senses get a little edgy when I see "doo".

I guess to explain why, just look at it this way. If an American with no prior knowledge of Japanese was to try and pronounce the above "doo", what would he typically say? Why, the phonetic equivalent to the "oo" in "moo" or "poo".

The "ou" in "dou" to this American would probably be more easy to attempt the correct sound. It's like the "ou" in "would", albeit more extended. Isn't there a French word "ou"? And it sounds awfully like the "dou", albeit a bit more accented, but closer than "oo" would ever be sounded out by our... typical American friend. biggrin

Here's how I see it.

The purpose of roomaji is not to actually have a fourth writing system to write Japanese in. Its only purpose is to be able, if needed, to write Japanese in a way that people who dont know the kana/kanji can read it. It's a shortcut to otherwise having to learn (at least) 2 new alphabets to be able to read the language.

So, since this is a shortcut specifically for those who DONT know the kana, and thus probably dont know correct pronounciation of the vowels or many words etc on their own...why would you want to write something with roomaji in a way that wont be read/pronounced correctly? Doesn't that defeat the purpose of shortcutting your way to being able to accurately read Japanese?

It may be all well and good to write 'doo' instead of 'dou' but that will only work if the person already knows that each 'o' should be pronounced individually and as a Japanese person would form the letter 'o'. When an English speaker sees 'doo' he/she automatically wants to say it like it rhymes with "moo".

Here's an example: If you're using, say, an English<->Japanese dictionary, and you've never really studied the language, but are trying to say somethings in Japanese, what matters is not how it is spelled but how you will end up pronouncing it. Saying "doomo arigatoo" as an English speaker would want to pronounce it will make it sound at best very silly, and at worse, very difficult to understand. Thus the use of the dictionary at all is negated if no one can understand you anyway.

And even if you do know enough to be perfect at the different pronounciations of the kana letters even when written with roomaji (which can take a while as you have to rewire the way your brain sees and processes the letters)...
When you do start using kana, you suddenly have to learn to spell it differently because it is spelled like 'dou' when using kana... So why change it in the first place?