Spinning top precess only when its axis is inclined

[Amith2006]

Sir,
Why does a spinning top precess only when its axis is inclined to the vertical direction of the gravitational field?

 

[Doc Al]

When the top spins at an angle to the vertical, gravity exerts a torque (via the reaction force at the support point) perpendicular to the top's angular momentum, which creates the precession. If the spinning top is perfectly vertical, there is no such torque and thus no precession.