It turns out that this unusual convention was a bit of an accident. In
Old and Middle English, the word for “I” was closer to its German
cousin, “ich,” and it was often spelled “ic.” At this point, the word
was not capitalized. However, the pronunciation changed over time and
so did the spelling, losing the consonant c.
At first, the new word, i, was left lowercase. However, it began to
grow taller than other words. It grew for a silly reason: a single
letter looks bad. Look at it: i. How sad. By the time Chaucer wrote
The Canterbury Tales in the late 1300s, I, the personal pronoun, was
slightly taller than its lowercase equivalent. From that point on, it
was typically capitalized.
The only other accepted single-letter word in English, a, is a larger
presence on the page. Its appearance isn’t as offensive as the thin i.
Today, though, some of us are regressing. In e-mails and instant
message conversations, capitalization conventions are backsliding.
http://blog.dictionary.com/whycapitali/