The gemara says בירא דשתית מיניה לא תשדי ביה קלא, do not throw a clod of earth into a well from which you drank. The Meiri says that this saying is directed at the treatment of others. Although a person should never belittle another, it is particularly abhorrent to belittle someone from whom he has received honor or service.
The Gra tells us an additional explanation in the adage of the gemara. It means that if a person finds pleasing material in a sefer, then he shouldn't deride other material in that sefer that he may not find pleasing. In fact, there is a saying, “Don’t bite the hand that feeds you.”