Why do we capitalize the word "I" ?

"England is where the capital “I” first reared its dotless head. In Old and Middle English, when “I” was still “ic,” “ich” or some variation thereof — before phonetic changes in the spoken language led to a stripped-down written form — the first-person pronoun was not majuscule in most cases. The generally accepted linguistic explanation for the capital “I” is that it could not stand alone, uncapitalized, as a single letter...

“Graphically, single letters are a problem,” says Charles Bigelow, a type historian and a designer of the Lucida and Wingdings font families. “They look like they broke off from a word or got lost or had some other accident.” When “I” shrunk to a single letter, Bigelow explains, “one little letter had to represent an important word, but it was too wimpy, graphically speaking, to carry the semantic burden, so the scribes made it bigger, which means taller, which means equivalent to a capital.”

The growing “I” became prevalent in the 13th and 14th centuries, with a Geoffrey Chaucer manuscript of “The Canterbury Tales” among the first evidence of this grammatical shift. Initially, distinctions were made between graphic marks denoting an “I” at the beginning of a sentence versus a midphrase first-person pronoun. Yet these variations eventually fell by the wayside, leaving us with our all-purpose capital “I,” a potent change apparently made for simplicity’s sake..." (Credit to the New York Times, via J-Walk)