What it means
どうして asks for the reason for something and means “why.” It usually comes at the beginning of a question, before the subject and the rest of the clause. It can be used in both familiar and polite questions; the question ending changes with the level of politeness.
Usage note
In familiar speech, a final の makes the question sound natural and explanatory, while のですか is polite. The ending のか is also possible, but it often sounds formal, written, or more serious than casual spoken Japanese.
How to use it
どうして妹は野菜を食べないのですか。
Why does the younger sister not eat vegetables?
どうして妹は野菜を食べないの。
Why does the younger sister not eat vegetables?
Example sentences
どうして今日は学校を休むのですか。
Why are you absent from school today?
どうして窓を開けたのですか。
Why did you open the window?
どうして図書館へ行くのですか。
Why are you going to the library?
どうして毎朝この道を歩くのですか。
Why do you walk along this road every morning?
どうして電気を消