んだもの

because…

→ の
phrase Common #7,208
because… Often used at the end of a sentence to give an emotional or emphatic reason, as if expecting the listener to understand or sympathize. The polite form is んですもの.
JA: だって、会いたかったんだもの。
EN: Because I wanted to see you.

あなた急にいなくなってしまうんだもの

You just disappear on me all of a sudden.

— When Marnie Was There (2014)

あの船は貨物船じゃなくて、漁船として造られたんだもの。

That ship was built as a fishing boat, not a cargo ship, after all.

だってお互いにもう話すことなんかないんだもの

Because we don't have anything left to talk about.

— 生きる (1952)

息子に会いに帰るんだもの

I'm going back home to see my son, after all.

— 戦争と女の顔 (2019)

こんなに乗ってるんだもの

I mean, there’s so much on it.

— 天空の城ラピュタ (1986)

だって、その船はスペンサー港に一度も入ったことがないんだもの。

Because that ship has never called at Spencer Harbor.

お母さんもおばあさんもおばあさんのおばあさんもみんなそうして来たんだもの

My mother, my grandmother, and my great-grandmother all did it that way.

— 天空の城ラピュタ (1986)

だって何年たっても――芽が出ないんだもの

Because no matter how many years pass, it still won't sprout.

— シカゴ (2002)

だって仕様がないじゃないの患者さんがあるんだもの

I can’t help it. I have patients to see, after all.

— 東京物語 (1953)

ちゃんと来たんだもの

But you did come, after all.

— When Marnie Was There (2014)

お煎餅すきなんだもの

I like rice crackers, after all.

— 東京物語 (1953)

あの子変わってるんだもの

That girl is just so weird.

— JUNO/ジュノ (2007)

君もみんなもこれまでよく頑張って来たんだもの。

You and everyone else have worked so hard all this time.

— となりのトトロ (1988)

あの石のおかげでぼくはシータに会えたんだもの

It was thanks to that stone that I got to meet Sheeta.

— 天空の城ラピュタ (1986)

だって好きなんだもの。

Because I like it.