Armor.

Looks are voluntary. But the full revelation of the subject in the face is not, as a rule, voluntary. Smiles are usually involuntary, and “gift smiles,” as one might call them, always so. Likewise laughter, to be genuine, must be involuntary—even though laughter is something of which only creatures with intentions, reason, and self-consciousness are capable. The important point is that, while smiling and laughing are movements of the mouth, the whole face is infused by them, so that the subject is revealed in them as "overcome". Laughing and smiling can also be willed, and when they are willed, they have a ghoulish, threatening quality, as when someone laughs cynically, or hides behind a smile. Voluntary laughter is a kind of spiritual armor, with which a person defends himself against a treacherous world by betraying it.

Sir Roger Scruton