When God reveals Himself in the world, the reaction of the Israelites stands in sharp contrast to the rest of the nations. While the nations of the world tremble in fear at the roaring thunder and flashing lightning, the Israelites hear those very same sounds and are filled with joy. They understand that these forces of nature are descending to execute justice upon the wicked [רד״ק]. This celebration is centered in Zion, the capital city where God's glory rests [רד״ק, אבן עזרא, ביאור שטיינזלץ], and spreads outward to the surrounding towns and villages of Judah [רד״ק, מצודת ציון, מאירי]. The location of this rejoicing is highly specific, as the ultimate judgment of the wicked is destined to take place nearby in the Valley of Jehoshaphat [רד״ק].
The exact nature of what triggers this joy is viewed from a few different angles. Some suggest that the people simply hear the rolling thunder itself [רד״ק]. Others explain that they receive news of God's vengeance against their enemies [רש״י, מצודת דוד], or hear that the leaders of the nations have finally bowed down in submission to God [אלשיך]. Regardless of the specific message, this reaction highlights a profound level of faith. While the surrounding nations refuse to surrender even when they see God's consuming fire with their own eyes, the people of Zion believe and rejoice merely from hearing the news [אלשיך].
Crucially, this joy does not stem from a desire for personal victory or selfish gain. Instead, it is a pure celebration of divine justice being realized in the world [מצודת דוד, ביאור שטיינזלץ, אלשיך]. These acts of judgment, which perfectly blend strict justice with mercy [אלשיך], serve as a public declaration that God is supreme and awesome, standing far above any other power in heaven or on earth [אבן עזרא, מאירי]. Furthermore, this joy reflects a deep understanding of how God guides the world. Even though Zion itself is directed by miraculous intervention that transcends the laws of nature, the people celebrate the fact that the rest of the nations are now being held accountable within the natural order, governed by true justice and law [מלבי״ם].