Fasting involves obedience, restraint, and the protection of character, not merely tolerating hunger and thirst. Lying, verbal abuse, or injuring others can undermine its formative purpose.
The words لَيْسَ لَهُ مِنْ صِيَامِهِ إِلَّا الظَّمَأُ strongly warn against being left with only the physical hardship. The expression calls attention to a serious loss, rather than praising physical deprivation.
If disagreement arises while fasting, restraining the tongue and avoiding injustice are practical ways to preserve the fast's intended ethical effect.