に夢中なんだ

be crazy about

→ に
phrase Rare #42,245
be crazy about A casual way to say that someone is very enthusiastic about or absorbed in something. なんだ adds an explanatory or emphatic tone; the subject is often omitted.
JA: 最近、彼女は日本のドラマに夢中なんだ。
EN: She's crazy about Japanese dramas lately.

サッカーに夢中なんだ

I'm crazy about soccer.

彼は化学に夢中なんだ

He's crazy about chemistry.

フットボールに夢中なんだ

I'm crazy about football.