当我们说某人 on top of the world 字面意思是这个人在世界顶端，但实际是说此人处于最快乐或最成功的境界。类似汉语里的幸福至极，心满意足。

例句

Winning a gold medal was the happiest moment of my life. I was on top of the world.

He's so happy now that he's recovered from his illness - he feels on top of the world.

Lucy has really bad mood swings. One minute she's on top of the world, and the next she's irritable and bad-tempered.

请注意

英语中有许多形容快乐的词语表达， 例如 on top, ecstatic, thrilled, delighted, happy as can be.

My daughter was thrilled when she got her exam results.

Amy was delighted when they offered her the job.

Ed's girlfriend is moving in with him. He's happy as can be.