If you write a small "i", it doesn't look as important as a capital "I". And remember, "I" is all about "you". :D

the word I is always capitalized because it refers to a person and acts like a name. Although it is only a pronoun, it is considered the most important one and the one that always get conjugated first in every verb conjugation. We take "I" very seriously, lol

It can be explained only historically, and since I've been teaching History of English, I'd say that Zerge's answer is quite correct. All others are just suppositions.

It's a one letter word and 'i' is the smallest letter of the alphabet. If you ever have to read an old manuscript or old printed book with spaces often omitted, you will understand that a small i could easily be read as the final letter of the previous word or the first letter of the next one. It has nothing to do with egoism. :)

It was strange for me because we don't use capital letter for 'I' in Turkish, if it isn't at the begining of the sentence. I don't know what the rule is for other languages. My opinion is for this subject that if something is unique in English its first letter is written as capital.