This might help:
LINGUIST List 9.253: Pronoun "I" again
Why do we write a capital letter for the 1st person of personal pronoun -I,whereas for the others we don't?
A quick Google search produced the following response: In Middle English, the first person singular was expressed with "ich", eventually shortened to "i" in the lower case. But printers encountered difficulties setting the lower case "i". The letter would be dropped unintentionally or run together with the words that followed or preceded it. So the original purpose of capitalizing the "I" was to make it stand out from other single letters and provide it with the status as a whole word...
I hope this answers your question!