underlie

Underlying describes something that lies under something else. This word has two different aspects:

First aspect - something provides the reason or basis of something else. Second aspect - something that might be hidden and difficult to see.

For example:

In order to find an effective solution to a problem, you have to understand all the underlying causes of the problem - not just the obvious causes.

The underlying reason for her anger last night is that she had a fight with her boss in the morning.