This week's Torah portion tells us that the voice of God was heard in the Tent of Meeting (between God and Moses). Rashi explains that this encounter was miraculous.
First, God's voice was only heard by Moses, not even by Aaron. Rashi didn't want us to think that others couldn't hear God's voice because it was too low, so he adds that the voice was as powerful as the one at Mount Sinai, at the Giving of the Torah.
Second, the voice filled the Tent of Meeting, but it didn't go beyond the entrance. Why wasn't God's voice heard outside the Mishkan (Tabernacle)? Surely that would have affected the people's understanding of God's holiness in observing His Will...
The answer is that God can make us fulfill mitzvot by overwhelming us with His holiness. Then we would all do His will. But this is not what HaShem wants. He wants us to Let's choose to do good for ourselves.
This is what Rashi teaches us by telling us that the voice of God in the Mishkan It was like the voice of God at Mount Sinai: after the Giving of the Torah, we no longer hear the voice of HaShem. This gives us the responsibility to choose to fulfill God's Will and not be forced by Divine revelation.
Adapted from Please tell me what the Rebbe said.