There must be a reason for that.

必然有原因啊。

youdao

There must be a reason for his absence.

他缺席一定有他的理由。

youdao

You can't just be sad. There must be a reason.

你不会只是难过。一定有一个理由。

youdao

If there is atmosphere in the passage I'm about to quote from Macbeth, it must be atmosphere of a certain kind and there for a certain reason.

如果下面这篇《麦克白》里的选段有所谓的“气氛“,那这种气氛一定因为某种原因以某种形式存在着。

耶鲁公开课 - 文学理论导论课程节选

There must be a reason.

NPR: Trip to Religious School Offers Window on Pakistan

There must be a compelling reason why they should hire you to manage and implement it, as opposed to using a current employee.

FORBES: Magazine Article

Lawyers who spoke at the debate said that the government's claim on retaining data contradicted the European Human Rights Act which states that there must be a good reason to interfere with personal data.

BBC: Extent of UK snooping revealed