Why is the pronoun ‘I’ the only pronoun that is capital by default in English?

Question

Why is the pronoun ‘I’ the only pronoun that is capital by default in English?

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Answer 1

Blame Charlemagne. He had vision problems so he was the one who insisted on lower case letters.

I is a proper pronoun. It’s a reference to a specific individual, namely the one speaking. You also used to be considered a proper pronoun but that was when You was a formal pronoun paired with the informal “thee/thy/thine” and it lost its status when the more casual pronoun disappeared.