Capital "I" image via Shutterstock

Some languages capitalize several of their pronouns. Some don’t capitalize any of them. English is an odd duck in that we only capitalize the first person singular, I.

Why? Honestly, we're not sure. Linguists and historians have been unable to find any record of a definitive explanation. We know this much: In Old and early Middle English, the German-flavored ich was used as the personal pronoun. Around the middle period of Middle English, personal pronouns proliferated and Ich, ich, Ic, ic, I and i were all used in writing with varying frequency. By the end of the Middle English period, I stood alone, tall and triumphant. The ch was dropped in one of the major phonetic changes that English experienced during these years, but the reason the solo i suddenly got the capital treatment is less clear. Here are some of the explanations scholars have proposed:

Capitalization might have been a linguistic concern. When I appears, it’s frequently the subject of the sentence, and may have gotten capitalized to denote its importance in a statement.
In a similar vein, capitalization might be psychological, affirming the importance not of the subject, but of the writer. I can confirm that we writers can be a self-obsessed bunch sometimes, but I can’t speak to the egotism of the scribes of the Middle Ages without a time machine. One problem with this hypothesis is that, if you’re going to capitalize I out of ego, why not do the same to every appearance of “me”?
Another explanation is that the capital I had less to do with language and more to do with the practicalities of handwriting. The lower case i looks a little weak on its own. Some historians - including Charles Bigelow, a type historian and designer of the Lucida and Wingdings font families - think that an i all by itself would have become illegible after multiple handlings and readings of a manuscript, and scribes had to make the pronoun graphically sturdier to stand the tests of time and smudging hands.