Explanation
Express the reason or cause, often used to justify something.
Formation
Verb-casual + んだものだ, い-Adjective + んだものだ, な-Adjective + なんだものだ, Noun + なんだものだ (often with から as んだものだから)
Detailed explanation
The ～だものだ grammar point is used to explain a reason or cause in a subjective, explanatory tone (e.g. 'because... after all'). Often, you'll see it used as ～だものだから ('because... after all').
Examples
疲れたんだものだから、休みたい。
Tsukareta n da mono da kara, yasumitai.
I want to rest because I'm tired.
お腹が空いたんだものだから、何か食べたい。
Onaka ga suita n da mono da kara, nanika tabetai.
I want to eat something because I'm hungry.
忙しいんだものだから、来られない。
Isogashii n da mono da kara, korarenai.
I can't come because I'm busy.
彼は親切なんだものだから、誰も彼が好きだ。
Kare wa shinsetsu na n da mono da kara, dare mo kare ga suki da.
Everyone likes him because he is kind.
Practice
Grammar content sourced from hanabira.org.