Fasting can cause a psychological sense of deprivation, which can cause one to grossly overeat after a fast. If that becomes habitual, it is a form of bulimia. Fasting can also aggravate other eating disorders. Fasting is a powerful medicine, to be taken in the right dose, as needed. Fasting will make you spiritually purer.
Many world religions use fasting as a tool for spiritual advancement.
The primary motivation of fasting for spiritual goals is that it clears the mind somewhat and may motivate or inspire people to do other spiritual practices. Real progress comes from other spiritual practices, not fasting. Further, the religions that use fasting also warn against excessive fasting.