You make an exclamation..or you could say it was a rhetorical question which does not require a response. A rhetorical question is a question that you ask without expecting an answer. The question might be one that does not have an answer. It might also be one that has an obvious answer but you have asked the question to make a point, to persuade or for literary effect.
'Why would anyone....' This phrase is used at the beginning of a criticism when you are drawing attention to something which irritates you, or appears ridiculous.
"Why would anyone wear purple socks!"
"Why would anyone go to the accident and emergency department of the hospital with such a minor problem as a sore throat!"
驚いたように言うか、または返事を求めないレトリックな質問のように言うこともできます。レトリックな質問というのは、返事を別に期待していない質問のことを言い、第一質問自体答えが用意されていない場合もあります。どう考えても答えがわかる質問を尋ねることで相手を説得したり、ポイントを際立たせる文学的手法でもあります。'Why would anyone....' このフレーズは、何か気に障ることや、ばかげているように見えることに対して批判的に述べるときに使われます。
例
"Why would anyone wear purple socks!"
"Why would anyone go to the accident and emergency department of the hospital with such a minor problem as a sore throat!"
Why are you wearing a pair of sunglasses at night?
サングラスのことはa pair of sunglasses と言うこともできます。レンズが2つありますからね。もちろんa pair of sunglasses は複数ではなく単数として扱います。
ボケを被せるのはどうでしょう？
Are you gonna erase somebody's memory like Men In Black?
メンインブラックみたいに誰かの記憶でも消す気なの？
Sunglasses are usually worn during the day when the sun can be bright and hurt your eyes it is not common to wear them at night or in the evening as it's darker and not so much light to hurt your eyes
Sometimes people suffer from a bad headaches (migraine) so might wear sunglasses to spot any light hurting them any more
Why do you feel the need to wear sunglasses at night?
Why are you wearing sunglasses when its dark?
"Why do you feel the need to wear sunglasses at night?" 'Why do you feel the need' is a polite way of expressing 'why'.
Why would you wear sunglasses when it is dark outside, often used as a jokingly way.
"Why do you feel the need to wear sunglasses at night?"（どうして夜なのにサングラスを掛けるのですか）
= 'Why do you feel the need' は 'why' の丁寧な言い方です。
"Why would you wear sunglasses when it is dark outside?"（何で外が暗いのにサングラスを掛けるの）は、しばしば冗談として使われます。
こんにちは。
このようなシチュエーションでは下記のような表現も使えます。
・Why are you wearing sunglasses at night.
「なんで夜にサングラスかけてるのよ」
・Sunglasses at night? Really?
「夜にサングラス？ほんとに？」
2例目は少し皮肉のある言い方です。
ぜひ参考にしてください。
日本 
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