Adam Clarke on Matthew 9:14

Thy disciples fast not? - Probably meaning that they did not fast so frequently as the others did, or for the same purposes, which is very likely, for the Pharisees had many superstitious fasts. They fasted in order to have lucky dreams, to obtain the interpretation of a dream, or to avert the evil import of a dream. They also fasted often, in order to obtain the things they wished for. The tract, Taanith is full of these fasts, and of the wonders performed thus by the Jewish doctors.