Why do we raise the tone of our voice when angry?

To some extent this is automatic. Anger is an emotion that causes increased adrenalin and this affects the muscle tone throughout our body including the muscles of our throat. A raised voice demands attention, and in anger, it is unattractive, in some way repellant, with the possibility, wittingly or unwittingly, of causing action in the other such as submission, retreat or apology.

But raising the voice suggests a psychological if not a physical distance, as if the angry person needs to reach another some way away. Think of the opposite, a lover, who whispers quietly in the ear. He feels close, accordingly, he doesn't want to raise his voice.