also: どおりで
adverb
no wonder; indeed; that explains it
Expresses that something is understandable or expected given a reason; often used when a cause becomes clear.
ああ、どうりで静かだと思った。みんな出かけていたのか。
Ah, no wonder it was quiet. Everyone had gone out.
彼は毎日練習している。
He practices every day. No wonder he's so good.
なるほど shows understanding or agreement with new information, while どうりで emphasizes that a reason explains a previously puzzling situation.
Derived from 道理 (reason, logic) + で (particle), literally 'by reason'. The kana spelling is now standard.