This is a powerful and direct request. It may be used by an adult to a youngster but may also be used the other way around.
'Being unreasonable' may mean being emotional. An emotional position has been taken and is only sustained by emotion. The code could equally have indicated: 'Don't be so emotional.'

'Being unreasonable' might also mean just producing a decision, request or order without showing any reasoned support.

'Being unreasonable' might also mean producing some very poor reasons for doing something. Or not following a logical chain of reason.

The code is a request to explain, to be logical and to be reasonable.

There are times when it is simply not possible to give reasons. A decision may be based on great experience or on intuition which cannot be itemized.

No one is obliged to provide reasons just because they have been requested. The code simply indicates that something appears to be 'unreasonable'.

The main use of the code may well be to switch from the emotion mode to the reason mode.