The gathering at Mount Sinai presented the Israelites with an experience beyond human limits, exposing them to direct Divine revelation. Caught between fear and awe, the people had to process this overwhelming event and determine how they would continue their relationship with God. This revelation was designed to prove God's existence and the reality of prophecy beyond any doubt. It served to completely disprove the claims of those who believed an infinite Creator could not communicate with flesh and blood. Through their own senses, the people witnessed two contrasting truths at once: God's power is entirely unlimited, yet He is capable of restricting Himself to speak directly with human beings [העמק דבר, מלבי״ם, ביאור יש״ר].
The primary approach among commentators is that the event consisted of different sensory dimensions. The visual aspect of the great fire represented His glory, while the intense sounds, lightning, and the blast of the horn reflected His greatness. The voice itself represented the actual content of the words spoken, which were the Ten Commandments [אבן עזרא, רבנו בחיי, ביאור יש״ר]. Even so, it is understood that the people did not witness the entirety of God's glory, but only a small fraction of it [שפתי כהן].
Experiencing this left the people in sheer astonishment that they had survived. Their original desire to see their King was granted by God specifically to cement their faith in Moses forever [רלב״ג, חתם סופר]. However, once they realized that prophecy was truly possible and the miracle had achieved its ultimate goal, a deep fear took hold of them regarding continuing to hear God in this direct manner.
There are several ways to understand the terror that gripped the people even after they survived the initial moments. One perspective suggests that their fear did not stem from the Divine speech itself, but rather from the blazing physical fire that accompanied it, which they worried would consume their souls [שד״ל, רקנאטי]. Another viewpoint explains this fear through the principle that God does not perform miracles without a purpose. Once the reality of prophecy was firmly proven, any further departure from the natural order through direct revelation was seen as both unnecessary and highly dangerous [מלבי״ם, חתם סופר].
A final perspective focuses on the spiritual condition of the people. Through three days of intense preparation and separation, the Israelites had elevated themselves to the highest possible spiritual peak of humanity. It was only because of this temporary, elevated state that they could endure the Divine speech and remain alive. Recognizing that they could not maintain this extreme level of separation as ordinary flesh and blood, they chose to have Moses continue as their mediator, as he remained in a constant state of purity [אדרת אליהו]. Ultimately, the sheer intensity of the revelation took a massive physical toll, leaving the people entirely exhausted and drained of their strength [שפתי כהן].