It takes away their potential to affirm life. Thus we not only give the person we pity the feeling to negate life, we also denigrate life when we pity, by pitying them we are saying 'isn't life sad'. Through pity we hurt them as well as ourselves, because pitying promotes the denial of life- the will to truth. Here Nietzsche revalues decadence to define the life denying practice of the will to truth, rather than their definition of decadence which to Nietzsche would be life affirming. Pity, not joy, becomes decadent.
Pity becomes tyranny.
Being compassionate is not the problem, though, being over-compassionate to the point of pity is the problem. Thus I advise those who have nothing to give. Thus we should not pity our students. Or makes them angry, and deny life. If we give them tools, they at least have the potential to be a fellow creator.