Ｉ don’t know how far Japanese language education has progressed in recent years, but back when I was a student, we were taught to use 「どうして」 for “why”. This was great until I went out into the real world and found out that people used another word that I was never taught: 「なんで」. Huh? What the-? Why wasn’t I taught the real deal instead of being handed down a second-rate word that nobody uses? Why eat a California roll when you can have real sushi?
Now that I know some more stuff (you know… stuff… and things…), I can think of two reasons: 1) 「なんで」 is a bit informal and would not be appropriate in some written contexts, and 2) 「なんで」 has some “issues” and they didn’t want to confuse the poor students (we were confused enough as it was).
Only in Japanese, can you write paragraphs explaining the word for “why”
I can understand why you would rather teach 「どうして」 as a teacher. It’s perfectly normal Japanese and you can use it just about anywhere in any context and be “safe”. The only problem is that Japanese people don’t use it that much. Why? Probably because 「なんで」 is shorter and easier to say.
Unlike 「どうして」, you have to be a little more careful when using 「なんで」. First of all, it’s more for conversational Japanese so you don’t want it use it on, for example, official documents. In this sense, 「なんで」 is kind of similar to “how come” instead of just “why”.
Second, things can get kind of confusing because 「なんで」 in kanji is 「何で」 and 「何」 is the kanji for 「なに」. 「で」 is also the particle for describing means. You can see the problem this overlapping might cause.
-A: With what (by what means) do you eat spaghetti?
-B: Why do you eat spaghetti?
The sentence above can have two meaning and there is no way to tell without any context. The dictionary says that 「何で」（なんで） means “why”. But if you write it in kanji, it looks identical to 「なにで」. There’s even a Japanese page with a survery of 「なんで」 vs 「なにで」. In general, 「なんで」 means “why” and 「なにで」 means “by what means” so when you want to make things absolutely clear, you should write it in hiragana.
In terms of ambiguity, 「どうして」 also has the same type of issues because 「どう」 means “how” and 「して」 is the te-form of 「する」.
-A: I don’t know what I should do (how to do so that it’s good).
-B: I don’t know why it’s good.
The first translation is more likely, but the second interpretation is possible as well. There’s no way to tell for sure without more context. (Aren’t you glad you didn’t pick an easy, sissy language to learn?)
Because two just isn’t enough
「何故」 is yet another word that means “why”, which we need because… I’m pretty sure there’s a good reason. 「何故」 is more formal than the other words for “why” and has, I feel, a sharper sting to it, if that makes any sense. It’s more suitable for when you want to ask hard-hitting questions such as, for example, a narrative for a documentary. As a result, you don’t hear this too often in regular, everyday conversations.
-Just why, exactly, did this happen?
Now that you know all the ways of saying “why” in Japanese (minus local dialects I’m not aware of), you can ask questions like the following in a variety of ways.
Why, Lord, why, is Japanese so complicated?!!
There, aren’t you happy now?