What it means
どうして asks for the reason for something and means “why” or “for what reason.” It normally appears before the clause being questioned. It is common in both familiar and polite questions, and the politeness comes from the ending of the question rather than from どうして itself.
Usage note
どうして asks about a reason, while どうやって asks about a method or means. Do not use どうして when you mean “how” in the sense of asking how to do something.
How to use it
どうして今日は早く帰るのですか？
Why are you going home early today?
どうして今日は早く帰るの？
Why are you going home early today?
Example sentences
どうして今日は学校を休むのですか。
Why are you taking the day off from school today?
どうして窓を開けたのですか。
Why did you open the window?
どうして電気を消したのですか。
Why did you turn off the light?
どうして駅まで走ったのですか。
Why did you run to the station?
どうして昨日の宿題をしなかったのですか。
Why did you not do yesterday's homework?