Musar על שבת 303:13

The Mishnah next lists the fact that no one was ever heard complaining that Jerusalem was so crowded that he could not find lodging for the night. This corresponds to Rabbi Akavyah's third awareness, that one needs to constantly be aware of before whom one will eventually have to give an accounting of one's conduct. We have already explained that different categories of people, even different categories of צדיקים, are measured in accordance with the spiritual level they had attained and are expected to maintain. When the Mishnah mentions Jerusalem, this refers not only to the physical Jerusalem in our world, but also to the spiritual concept of what our sages have called ירושלים של מעלה, the Jerusalem in the Celestial Regions. Torah scholars, as we mention in our prayers daily, מרבים שלום בעולם, constantly add new dimensions to the concept of peace and harmony in the world. They do this by joining together different parts of the אהל של תורה, the tent which is a symbol of Torah.