Why did the high priest's robe need bells at all? The Targum Pseudo-Jonathan on Exodus 28:35 gives the quiet, terrifying answer. Its voice shall be heard at the time that he hath entered the holy place before the Lord, and at the time that he cometh out, that he die not by the flaming fire.
The sanctuary was not safe. Enter without warning — walk in as though the Holy of Holies were a living room — and the holiness itself became combustible. The bells were not ceremonial music. They were a knock. The high priest rang himself into the presence of God the way a child might call out before opening a parent's door.
The Sages took the detail as a theology of respect. Even Aaron, anointed and ordained, could not barge. Holiness responds to those who announce themselves. The same principle, the rabbis taught, governs prayer: a person does not rush into the standing Amidah without first stepping three steps back and three steps forward, announcing intention before speaking a word.
The takeaway is that the distance between casual and sacred is not attitude. It is process. Ring the bell. Wait. Enter.