Simple terms without any fancies. If I tilt the disc forward in order to allow the rotor thrust to initiate the forward movement, the airframe will not only go up as before the forward initiation on the cyclic but will also tilt the nose downwards towards the ground. If I do not increase collective in order to increase the rotor thrust to compensate for the tendency to fly forward and downwards, i will fly into the ground as the original applied rotor thrust is now divided between lift and forward movement and thus has to be increased in order to maintain height initially.