When your interest in something / someone cools down / off, you become less enthusiastic about it / them.
- Before he even realized it, his interest in his master’s daughter cooled off.
- My father always knew that my interest in horse racing would soon cool down.
Cool down and cool off can also be used to talk about becoming less angry.
- It took him quite some time to cool off.
- There is no point in talking to him when he is very angry. Let him cool off.
- Cool down. There is no point in getting angry.