I'm no expert the answer must lie in the relationship between the aircraft's centre of gravity and the aerodynamic centre of lift (c:g ahead of the c:l and the tail must have a downforce and vice versa).
More simply though, consider an aircraft's undercarriage configuration.
With a tricycle configuration the aircraft's c:g lies ahead of the maingear (else it'd fall over!) so there must be a downforce on the tailplane to rotate the aircraft into the climb attitude. Coversely, with a tail-dragger the c:g lies behind the maingear (else it'd fall over the other way!) so the tailplane must create lift to raise the tail.
I'm sure there will be exceptions to the rule - watch a B52 take off and you'd suspect the tailplane was producing lift.
As I said, I'm no expert - it could all be BS.
MT