Parsha Mitzvot: Acharei Mot: Mitzvah 186 – Concept 324
“And God said to Moshe: Speak to Aharon, your brother, he shall not come at all times into the Sanctuary, within the Curtain, in front of the Cover that is upon the Ark, so that he shall not die; for in a cloud will I appear upon the Ark-cover (Vayikra 16:2).” A Kohen must not enter the Sanctuary indiscriminately (Rambam, Hilchot Bi’at Mikdash – The Laws of Entering the Temple).
All those who enter the Sanctuary must appreciate that it is a place of deep intimacy between God and the people; just as one would not walk into the room of people being intimate, so too, one may not enter the Sanctuary unless an active participant in the intimacy; the Service. (Ir Miklat)
Our behavior in a synagogue reflects the level of intimacy we believe we achieve through our prayers.