A veil separated the Holy Place from the Most Holy Place. The sanctuary was like God's palace. The Most Holy Place was his throne room, and the ark was his throne (see Leviticus 1:1). In the ancient Near East, entering a king's presence without an invitation could mean death (Esther 4:11).
So, Aaron could not enter the Most Holy Place informally. Even the high priest could not enter. The only exception was when God required his presence for the Day of Atonement ceremony.