He's madly in love with his daughter so he's always thinking about her.
He's crazy about his daughter, she's the only thing he ever thinks about.
この場合、次のような言い方ができますよ。
ーHe's madly in love with his daughter so he's always thinking about her.
「彼は娘に[メロメロ](https://eikaiwa.dmm.com/uknow/questions/100893/)でいつも娘のことを考えている」
to be always thinking about ...「いつも…について考えている」＝「頭の中は…のことでいっぱいである」と言えます。
ーHe's crazy about his daughter, she's the only thing he ever thinks about.
「彼は娘にメロメロで、娘のことしか考えていない」
... is the only thing one ever thinks about「…が人が考える唯一のこと」＝「頭の中には…しかない」と言えます。
ご参考まで！
カナダ 
