Why is Tanjiro’s forehead so hard?

Why is Tanjiro’s forehead so hard? The major literary reason for Tanjiro having such a hard head is to be symbolic of his determination and fighting style. Time and time again, Tanjiro is shown to press forward and fight on even when all hope seems lost.

The major literary reason for Tanjiro having such a hard head is to be symbolic of his determination and fighting style. Time and time again, Tanjiro is shown to press forward and fight on even when all hope seems lost.
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