Textbook vs Real
Textbook당신 때문입니다dang-sin ttae-mun-im-ni-da
Real너 때문이야neo ttae-mun-i-ya
Why Koreans say it this way
Accusatory finger-pointing. 너 (you, casual/rude) vs 당신 (you, formal). 이야 (casual copula) vs 입니다 (formal copula). In real confrontation, nobody uses formal speech — it would sound sarcastic.
From: 더 글로리 EP4