Good question! Not much written about it--even the Oxford English Dictionary doesn't address it. However, they do on dictionary.com and they say that it began to be capitalized in the mid 13th century "to mark it as a distinct word and avoid misreading in handwritten manuscripts." I assume from then on it just became normal in English. Odd though since no other pronoun is capitalized! Maybe philosophically that points out the tremendously strong urge within ourselves to be selfish and self-exalting (not a good trait!!).

Hello. 'i' is always written as a capital when used on its own. This is just a rule of the English language.