Repaying hostility with kindness is not an act of surrender, but a brilliant psychological and spiritual strategy. Offering sustenance to an enemy completely overturns the expected power dynamic, granting the injured party a twofold advantage.
The primary approach among commentators is that extending charity to an enemy is akin to scooping up burning coals from a fire and placing them directly on his head. Rather than causing physical pain, this unexpected generosity inflicts a profound sense of shame and embarrassment. When a hostile person is forced to accept help from the very individual he has wronged, the memory of that kindness burns him from the inside. It breaks down his stubbornness and effectively stops him from causing further harm [אבן עזרא, מצודת דוד, מלבי״ם, עמנואל הרומי]. In this way, doing good serves as a sweet and highly effective form of revenge, striking back at the enemy through pure generosity [ביאור שטיינזלץ]. Another perspective highlights that just as fire consumes, this profound act of kindness completely neutralizes and destroys the threat posed by the enemy [אמרי דעת].
Following this act of unexpected goodness, God promises a reward. The primary approach among commentators is that God grants a generous reward simply for fulfilling the Commandment of charity, even if the underlying motive included a desire to shame the enemy or exact a measure of revenge. On the other hand, a different interpretation suggests that God's payment is the restoration of peace. He will either transform the enemy into a friend or subdue him entirely, ensuring he can no longer pose a threat [רש״י, עמנואל הרומי].
On a deeper, allegorical level, this dynamic represents the internal human struggle against the evil inclination, which is viewed as a person's greatest enemy. In this context, hunger and thirst symbolize material desires and negative thoughts, while bread and water represent the Written and Oral Torah. Immersing oneself in Torah study acts as a consuming fire that burns away and nullifies the evil inclination. This is especially true when a person takes preventive action and turns to study at the very beginning—or the "head"—of a temptation. Even if the sole motivation for studying is to escape sin and defeat this internal enemy, God still provides a full and complete reward for the effort and energy spent in that spiritual labor [אלשיך, עמנואל הרומי].