Like many things in English, it’s not based on any logical rule.
Early writers simply thought the letter i looked too small by itself.
The only other single-letter word in English is “a,” which doesn’t carry a lot of semantic weight.
Writers started capitalizing “I” to give it more importance, and it became tradition.
In Old English, it used to be “ich” or “ic” and it was not capitalized. Other pronouns like “him” and “her” follow this pattern. I guess you could say “I” is special.
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