expression, ichidan verb
lose interest; cool down; grow less enthusiastic
Metaphorical expression: one's passion or enthusiasm cools off, like a fever subsiding. Often used for hobbies, projects, or relationships.
最初は夢中だったけど、最近熱が冷めてきた。
I was obsessed at first, but lately my enthusiasm has cooled.
彼女への熱が冷めるまで時間がかかった。
It took time for my feelings for her to fade.