Lesson 14: Why, cruel world? なぜ なんで どうして
なぜ, なんで, and どうして mean "why". They can also be used the same way as "how". "Tone of voice" questions with these words don't need a ？.
Generally, the longer something is, the more polite it is, but in very formal situations you can use the short things again. This works here, too. どうして is the most polite, なんで is neutral, and なぜ is the most simple.
なんで来ません So, why won't he come?
どうして遅いの？ Why is it late?
なぜだろう I wonder why. (not sarcastic)
These words are used in different places by different people in different registers of speech. Sometimes one is more frequent than another in a certain phrase, but there's no real pattern. なぜ can sound rude in conversation.