אמר רבי תנחום בר חנילאי: בנוהג שבעולם משוי שקשה לאחד נוח לשנים, ולשנים נוח לארבע, או שמא משוי שקשה לס' רבוא, נוח לאחד?! כל ישראל עומדים לפני הר סיני ואומרים (דברים ח'): אם יוספים אנחנו לשמוע וגו', ומשה שומע קול הדבור עצמו וחיה?!תדע לך שהוא כן, שמכולן לא קרא אלא למשה, לכך נאמר: ויקרא אל משה
Achronim ask that the metaphor is invalid. A physical burden carried by two is easier, because each person is only carrying half. By Mattan Torah, the number of listeners is irrelevant, because every one of them is hearing the entire thing individually- sharing the experience does not lessen the experience at all.
So Reb Dovid Soloveichik (Shai laTorah III) and Reb Berel Povarsky (Bad Kodesh here) say the same answer- that you see from here the power of a tzibur, that bearing the spiritual thing together makes it easier for each, or that it's too esoteric to explain, or something along those lines. Reb Dovid's words, as quoted there-
ומבואר במדרש דלא כן הפשט, אלא שהיה זה "קול רוחני" אשר כל כלל ישראל "כאיש אחד בלב אחד" היו שומעים אותו ומשו"ה נקרא "משוי של ששים רבוא" והבן הדבר כי עמוק הוא
What does it mean that Klal Yisrael couldn't hear the voice of the Ribono shel Olam? It clearly wasn't a matter of physical force, because then it wouldn't be a gadlus that Moshe could bear it. It must be a spiritual thing. What was it about hearing the Ribono shel Olam that was so spiritually challenging?
The Rambam in Peirush haMishnayos at the end of Makkos says that in a lifetime of doing thousands of mitzvos every day, we cherish the hope that once in a lifetime we might do one mitzva perfectly. My father, zatzal, for example- in Samarkhand, he once gave a man every penny he had, and then gave him his shoes. He decided that he would find a way to survive, while the other man was so desperate he needed everything my father could give him. That was a pure and perfect mitzva of tzedaka. But this is a rare, rare thing.
When the Ribono shel Olam gave us the Torah, it was an unimaginably powerful exposure to the kedusha of the Torah and the perfection of the Ribono shel Olam. The people said that this is beyond their ability. When you get a perfect vision of what a mitzva is, and what kedusha is, then you feel that you have to do every single mitzva, every day, perfectly. But the reality is that such a thing is not possible for flesh and blood. It seemed like they would need to be not just malachim, but greater than malachim. On the other hand, Klal Yisrael as a whole includes individuals that dedicate their whole lives to a particular mitzva. You have the masmidim whose every breath is limud haTorah. You have others that run after poor people to help them and encourage them. You have others that wear fifty talleisim for a perfect mitzva of tzitzis. At our Kiddush this morning, we hosted Rabbi Abraham Cooper of the Weisenthal Center, and we realized, once again, what it means to totally and absolutely dedicate one's life to advocacy for Klal Yisrael. Within Klal Yisrael as a whole, perhaps every single mitzvah can be fulfilled as it should be. But when we saw Mattan Torah, and we heard the mitzvos from Hashem, we felt that even with the din of areivus we cannot possibly do what needs to be done. Even if the burden was shared, the mitzvos could not be fulfilled as perfectly as we saw they ought to be fulfilled, and we despaired, perhaps we were even terrified by the expectations, and we couldn't survive facing the contrast between our duty to Hashem's perfect plan and our hopeless human failings and inadequacy.