English capitalization is a can of worms nestled inside the larger swamp of English spelling -- i.e, there is almost never any good reason for why anything is spelled, punctuated, or capitalized any particular way -- they just are. Sometimes.
However, one contributing reason for this particular feature is probably the fact that both I and O are usually stressed (as one can tell from the fact that they contain diphthongs, which require stress to avoid reduction), while a is never stressed and therefore always reduced to /ə/.
Capitalization (especially of titles) is a tricky business in English. There is no hard-and-fast rule about capitalization. In fact, many of the major style guides disagree on exactly what should or shouldn't be capitalized. However, I'll give some general guidelines and let you make up your own mind:
Titles in direct address
In all major style guides, when a title is used as a name in direct address of that individual, it is capitalized. For example: "That was very kind of you Mom." "I love you Dad." "Stop being so good to me, Master." "So, I have to have surgery, Doctor?"
Titles used in direct reference
When referring to a specific individual with their title, the title should be capitalized. For example: "I was excited when President Barrack Obama walked in the room." "My favorite comic book character is Mr. Tony Stark." "It took a while, but we finally got in to see Doctor John Smith."
However, (somewhat less commonly) some prefer that occupational titles not be capitalized, unless abbreviated. For example, "Dr. John Smith" and "doctor John Smith".
Titles used in indirect reference
Most style guides prefer titles used in reference to be lowercase. For example, "I love my mother." "The president gave a great speech." "My doctor did a great job with my surgery."
However, some titles are exempted from this. God (when referring to the Judeo-Christian god) is always capitalized. The King and the Queen are often capitalized. (For example, "The Queen has quite a way with words.") When referring to a specific office, there is disagreement about what should or should not be capitalized. Chicago and Wikipedia prefer "the president of the United States", while NY Times prefers "the President of the United States". However, everyone prefers "King of France" to "king of France".
Best Answer
One of the capitalization rules is, to capitalize the letter "I" when referring to yourself, so it must be capitalized:
So, it would be:
as well as:
N.B. Only "i" when used as a pronoun is capitalized.