原因何在?

What is the reason for it?

深究原因何在

get to the bottom of it

原因何在？

What is the reason for it?

本合同可在任何时间因任何原因修改？搞笑呢！

Контракт может быть изменен в любое время по любой причине... Вот те на!

原因在于

the reason lies in ...; причина в том, что

真正的原因在于他事事要随心所欲。

The true cause lay in the fact that he would have everything his own way.

事物发展的根本原因在于其内部矛盾。

The fundamental cause of the development of a thing lies in its internal contradictoriness.