why is me lowercase, while I is uppercase?

According to Wikipedia (and Wikipedia is, of course, God), capitalization began around 1250 when "ic" was shortened to "i," and because "i" was apparently too small to look like its own word, they capitalized it.

Back in those times, a capitalized "I" took up more space than it does now, because it was written in serif font (so it had all the extra little wing-things, like this: http://www.identifont.com/samples/adobe/StoneSerif... ) and forget about when they wrote in calligraphy, it was huge (like this: http://www.fromoldbooks.org/Brown-LettersAndLetter... ).

German does the same thing (they capitalize "Ich" but not "mich"), and since German and English have a common ancestor, that make sense.

Edit: True, good point. Perhaps "a" isn't as important to the meaning of the sentence as "I" is?

I would guess it's because I is a proper noun, but if the head of your English department didn't come up with that...I'm probably not right.

not sure. it's just one of those weird rules that nobody follows while they don't know why. it's been like that since the formation of english. it's like asking how french people came up with the genders for their words. basically, the answer is because.