I get the historical explanations, but I was always taught in school that “I” is a proper pronoun. While that is probably fabricated by a low budget public education system, it does make for a fun explanation. The basic principle is that you are naming yourself in the first person. Since we rarely refer to ourselves in the third person we capitalize our own personal pronoun.
Again, probably not the real reason, but it was what I heard in English classes until maybe the 8th grade or so.
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