Why does a field dependency auto choose the 1st field in picklist when set as mandatory?

Why does a field dependency auto choose the 1st field in picklist when set as mandatory?

I have a pick list in the cases module called "Warehouse" with 2 options DC 1 or DC2.  I also have a field dependency for Case reason = Mispick which will make the picklist a required field however the picklist auto defaults to DC1 (Even though the default value is "None".  A user can still select either DC1 or DC2 however my fear (From long experience), is that our customer service agents will not even notice the "Warehouse" field because it auto selected DC1 and allowed them to save the case record.  In salesforce: This same programing would leave a blank space but make the field required to save the record so the customer service agent would have to consciously choose the DC