If the FO has only ever been told by the captain, we are diverting this way and doing this and offers up no explanation as to why then how is the FO going to learn.
Certainly there will be circumstances when tiime doesn't permit an explanation but why not explain after the flight. "This is what I was thinking and why"
The FO is there to learn as much as they can from the captain so that when their time comes for an upgrade they will be far better prepared for it.