The letter I is capitalized in an English sentence when it is used as a word to refer to the person writing. Proper (personal) names of people, places, and things are capitalized in English grammar. The word "I" is a personal pronoun; its function is to take the place of the name of the person doing the writing.

If it's not capitalised, we simply miss it in reading. It's a single letter, remember?

I think we lost that knowledge, maybe it is just a shared wishful thought to express that 'I am important'. In my language (Italian) we say /or/ with the very humble /o/, and a lot of people are replacing it with the wrong term /piuttosto/ just to make it sound big! Actually I think Peachey is right, but I am just guessing.

I have always believed that what Peachey said is right. It began to be capitalized c.1250 to mark it as a distinct word and avoid misreading in handwritten manuscripts. with thanks to etymology online.com for that little quote.

Why not he, she, it, we, they and just I? Good question. I think because it looks better when capitalized.