On Fasting and Spiritual Purpose

כל היושב בתענית נקרא חוטא )תענית יא(

One who “sits” in a fast [someone who fasts for self-affliction] is called a sinner (Ta'anis 11:1)

If one “sits still” while he fasts - if he fasts but makes no spiritual progress and the fast has no deeper effect on him - he has afflicted himself without purpose and therefore is considered a sinner.

Kesav Sofer

The Sefer Beis Ahron, quotes Reb Shlomo Karliner ZY”A in which he states if there is a purpose in fasting many Ta'aneisim. If he (the faster) is on the correct path, good. If not, why is he punishing his stomach as he will only be more crooked. And to fast more than obligated is only if it will not take away from Avodas Hakodesh, Davening and Learning.