The gemara (Shabbat 88a) derives from the pasuk, "They stood at the bottom of the mountain" (Shemot 19:17) that Hashem actually held the mountain over them. Remember that this is after the whole discussion between Moshe and Bnei Yisrael and after Bnei Yisrael unanimously said "na’aseh v’nishma" (we will do and hear). Thus, it is unclear what changed that brought Hashem to coerce the nation to accept the Torah by saying that if they do not accept it, they would be buried under the mountain.
While many explanations have been said it seems to me that the following one has merit as well. There is a difference between the power that one has to decide something and the power he has to carry it out. Theoretically, these two powers are directly connected, as that which one wants, he carries out. However, in practice, there can be quite a distance between what one decides and what one does, as some good things remain a good wish and no more. Execution requires effort, sacrificing comfort, and overcoming difficulties, which are harder than just wanting something.
We remember well the days of the struggle against the British Mandate before the State of Israel was established. It was a very, very difficult struggle and included a tremendous amount of discomfort. However, the Jewish community stood up to the challenges in a very impressive way. The important thing to us was reaching the goal of statehood. In contrast, since the time we achieved independence, we often see laxness under pressure and a weakening of resolve, causing many goals to remain unachieved. It is not that it is impossible to achieve what we want, but that we have lost some of our stubbornness. What is behind the change?
The biggest difference is that then, as we used to like to say, we had a secret weapon called ein bereira (there is no alternative). As they used to say in the Navardok Yeshiva: "When you can’t do something, you have to do it; and when you have to, then you can do it."
The same thing is true regarding the giving of the Torah. The saying of na’aseh v’nishma was fully sincere, but expressed only proper good will. However, Hashem knew human nature and knew that before too long Bnei Yisrael would be dancing around a Golden Calf, because there is a big distance between deciding and succeeding to carry out. It is easier to dance around a Calf than to "’kill oneself’ in the tent [of Torah]." It takes Divine Assistance, and that is what Hashem provided with a little bit of coercion, in the form of a "mountain over their heads." He showed them that it is not only their desire to accept the Torah but that there is no other way. The entire existence of the Nation of Israel depends upon the extent to which they keep the Torah. This feeling of no alternative is what allows the good will to triumph in practice.