Modern English ultimately derives from Old English, which is a Germanic language. Up until the 14th century, the first person pronoun in English was ich. When the old English "Ich" (like in German) became reduced to "i" it was thought to be too small and insignificant to be a real word and could easily get attached to the end or beginning of another word. So, "i" was capitalized to "I" to avoid misreading handwritten manuscripts. And now it is a grammar rule.